It was one of the worst mass shootings ever at a military base in the United States.

The gunman, who was still alive after being shot four times, was identified by law enforcement authorities as Maj. Nidal Malik Hasan, 39, who had been in the service since 1995. Major Hasan was about to be deployed to Iraq or Afghanistan, said Senator Kay Bailey Hutchison, Republican of Texas.

Clad in a military uniform and firing an automatic pistol and another weapon, Major Hasan, a balding, chubby-faced man with heavy eyebrows, sprayed bullets inside a crowded medical processing center for soldiers returning from or about to be sent overseas, military officials said.

The victims, nearly all military personnel but including two civilians, were cut down in clusters, the officials said. Witnesses told military investigators that medics working at the center tore open the clothing of the dead and wounded to get at the wounds and administer first aid.

As the shooting unfolded, military police and civilian officers of the Department of the Army responded and returned the gunman’s fire, officials said, adding that Major Hasan was shot by a first-responder, who was herself wounded in the exchange.

In the confusion of a day of wild and misleading reports, the major and the officer who shot him were both reported killed in the gun battle, but both reports were erroneous.

Eight hours after the shootings, Lt. Gen. Robert W. Cone, a base spokesmen, said Major Hasan, whom he described as the sole gunman, had been shot four times, but was hospitalized off the base, under around-the-clock guard, in stable condition and was not in imminent danger of dying.

Another military spokesman listed the major’s condition as critical. The condition of the officer who shot the gunman was not given.

Major Hasan was not speaking to investigators, and much about his background — and his motives — were unknown.

General Cone said that terrorism was not being ruled out, but that preliminary evidence did not suggest that the rampage had been an act of terrorism. Fox News quoted a retired Army colonel, Terry Lee, as saying that Major Hasan, with whom he worked, had voiced hope thatPresident Obama would pull American troops out of Iraq and Afghanistan, had argued with military colleagues who supported the wars and had tried to prevent his own deployment.

As a parade of ambulances wailed to the scene of the shootings, officials said the extent of injuries to the wounded varied significantly, with some in critical condition and others lightly wounded. General Cone praised the first-responders and the medics who acted quickly to administer first aid at the scene.

“Horrible as this was, I think it could have been much worse,” the general said.

The rampage recalled other mass shootings in the United States, including 13 killed at a center for immigrants in upstate New York last April, the deaths of 10 during a gunman’s rampage in Alabama in March and 32 people killed at Virginia Tech in 2007, the deadliest shooting in modern American history.

As a widespread investigation by the military, the F.B.I., and other agencies began, much about the assault in Texas remained unclear. Department of Homeland Security officials said the Army would take the lead in the investigation.

A federal law enforcement official said the F.B.I. was sending more agents to join the inquiry. On Thursday night, F.B.I. agents were interviewing residents of a townhouse complex in the Washington suburb of Kensington, Md., where Major Hasan had lived before moving to Texas.

Mr. Obama called the shootings “a horrific outburst of violence” and urged Americans to pray for those who were killed and wounded.

“It is difficult enough when we lose these men and women in battles overseas,” he said. “It is horrifying that they should come under fire at an Army base on American soil.”

The president pledged “to get answers to every single question about this horrible incident.”

Military records indicated that Major Hasan was single, had been born in Virginia, had never served abroad and listed “no religious preference” on his personnel records. Three other soldiers, their roles unclear, were taken into custody in connection with the rampage. The office of Representative John Carter, Republican of Texas, said they were later released, but a Fort Hood spokesman could not confirm that. General Cone said that more than 100 people had been questioned during the day.

Fort Hood, near Killeen and 100 miles south of Dallas-Fort Worth, is the largest active duty military post in the United States, 340 square miles of training and support facilities and homes, a virtual city for more than 50,000 military personnel and some 150,000 family members and civilian support personnel. It has been a major center for troops being deployed to or returning from service in Iraq and Afghanistan.

The base went into lockdown shortly after the shootings. Gates were closed and barriers put up at all entrance and exit checkpoints, and the military police turned away all but essential personnel. Schools on the base were closed, playgrounds were deserted and sidewalks were empty. Sirens wailed across the base through the afternoon, a warning to military personnel and their families to remain indoor.

Military commanders were instructed to account for all personnel on the base.

“The immediate concern is to make sure that all of our soldiers and family members are safe, and that’s what commanders have been instructed to do,” said Jay Adams of the First Army, Division West, at Ford Hood.

General Cone said the shooting took place about 1:30 p.m., inside a complex of buildings that he called a Soldier Readiness Processing Center. The type of weapons used was unclear, and it was not known whether the gunman had reloaded, although it seemed likely, given that 43 people were shot, perhaps more than once.

All the victims were gunned down “in the same area,” General Cone said.

As the shootings ended, scores of emergency vehicles rushed to the scene, which is in the center of the fort, and dozens of ambulances carried the shooting victims to hospitals in the region.

Both of the handguns used by Major Hasan were recovered at the scene, officials said. Investigators said the major’s computers, cellphones and papers would be examined, his past investigated and his friends, relatives and military acquaintances would be interviewed in an effort to develop a profile of him and try to learn what had motivated his deadly outburst.

Major Hasan was assigned to the Darnall Army Medical Center at Fort Hood.

The weapons used in the attack were described as “civilian” handguns. Security experts said the fact that two handguns had been used suggested premeditation, as opposed to a spontaneous act.

Rifles and assault weapons are conspicuous and not ordinarily seen on the streets of a military post, and medical personnel would have no reason to carry any weapon, they said. Moreover, security experts noted, it took a lot of ammunition to shoot 43 people, another indication of premeditation.

It appeared certain that the shootings would generate a whole new look at questions of security on military posts of all the armed forces in the United States. Expressions of dismay were voiced by public officials across the country.

The Muslim Public Affairs Council, speaking for many American Muslims, condemned the shootings as a “heinous incident” and said, “We share the sentiment of our president.”

The council added, “Our entire organization extends its heartfelt condolences to the families of those killed as well as those wounded and their loved ones.”

General Cone said Fort Hood was “absolutely devastated.”

News of the shooting set off panic among families and friends of the base personnel. Alyssa Marie Seace’s husband, Pfc. Ray Seace Jr., sent her a text message just before 2 p.m. saying that someone had “shot up the S.R.P. building,” referring to the Soldier Readiness Processing Center. He told her he was “hiding.” Ms. Seace, 18, who lives about five minutes from the base and had not been watching the news, reacted with alarm. She texted him back but got no response. She called her father in Connecticut, who told her not to call her husband because it might reveal his hiding place. Finally, 45 minutes later, her husband, a mechanic who is scheduled to deploy to Iraq in February, texted back to say that three people from his unit had been hit and that a dozen people in all were dead. By late afternoon, the sirens at Fort Hood had fallen silent. In Killeen, state troopers were parked on ridges overlooking the two main highways through town. In residential areas, the only signs of life were cars moving through the streets. In the business districts, people went about their business. In 1991, Killeen was the scene of one of the worst mass killings in American history. A gunman drove his pickup truck through the window of a cafeteria, fatally shot 22 people with a handgun, then killed himself.

Fort Hood, opened in September 1942 as America geared up for World War II, was named for Gen. John Bell Hood of the Confederacy. It has been used continuously for armor training and is charged with maintaining readiness for combat missions It is a place that feels, on ordinary days, like one of the safest in the world, surrounded by those who protect the nation with their lives. It is home to nine schools — seven elementary schools and two middle schools, for the children of personnel. But on Thursday, the streets were lined with emergency vehicles, their lights flashing and sirens piercing the air as Texas Rangers and state troopers took up posts at the gates to seal the base. Shortly after 7 p.m., the sirens sounded again and over the loudspeakers a woman’s voice that could be heard all over the base announced in a clipped military fashion: “Declared emergency no longer exists.”

The gates reopened, and a stream of cars and trucks that had been bottled up for hours began to move out.

That was just one of many small and enigmatic details to emerge Friday about Major Hasan, the 39-year-old Armypsychiatrist accused of a shooting spree at Fort Hood that killed 13 people Thursday and wounded at least 30 others.

An American-born Muslim of Palestinian descent, he was deeply dismayed by the wars in Iraq and Afghanistan but proud of his Army job. He wore Middle Eastern clothes to the convenience store and his battle fatigues to the mosque. He was trained to counsel troubled soldiers, but bottled up his own distress about deploying.

Gen. George W. Casey Jr., the Army chief of staff, andJohn M. McHugh, the Army secretary, traveled Friday to Fort Hood, the Army’s largest post, as a widespread investigation into the shooting began.

“This is a tough one,” General Casey said at a news conference. “It’s a kick in the gut. There’s no doubt about that.”

The local police said that ballistics tests showed there was only one gunman and that none of the casualties had been hit by bullets fired by the police.

But the military and federal investigators pointedly refused to release further details on how the shootings happened, why there were initial reports of multiple attackers and why officials took several hours to correct news media reports that Major Hasan had been killed.

Most significant, officials were not prepared to say whether the attack was the act of a lone and troubled man or connected to terrorist groups, foreign or domestic.

President Obama asked the nation to avoid “jumping to conclusions” while the investigations into the Fort Hood rampage continued.

Major Hasan was shot four times during the attack. On Friday, officers at Fort Hood reported that he had been moved to Brooke Army Medical Center in San Antonio for security and medical reasons.

Senator Kay Bailey Hutchison, Republican of Texas, said Army officials were trying to determine “if there is something more than just one deranged person involved here.”

Ms. Hutchison said in remarks at the base that while Major Hasan was the only one who had opened fire, it was still unclear whether he had planned the attack alone. “That is a question still to be asked,” Ms. Hutchison said. “That is not a question that has been resolved.”

She also said that the shooting had prompted Army officials to examine procedures in tracking people who may have problems.

“Was enough done?” she said. “I don’t think that anyone would have ever expected a psychiatrist trained to help others’ mental health would be the one who would go off himself, unless there’s more to it, and that’s what they’re looking for.”

In Washington, a law enforcement official said an early search of Major Hasan’s computer did not indicate any direct exchanges with known terrorists. The official said investigators did not have a complete record of Major Hasan’s Internet use, as he had multiple e-mail accounts and used computers in several locations.

The Federal Bureau of Investigationbecame aware earlier this year of Internet postings by a man calling himself Nidal Hasan. The postings drew attention because they favorably discussed suicide bombings. But the investigators are still not clear as to whether the writer was Major Hasan.

Whether investigators conclude that Major Hasan acted alone — so that the crime was purely military-on-military — or whether they uncover evidence of any civilian co-conspirators off the base will help determine whether he faces trial by court-martial or in federal court.

Under either civilian law or the Uniform Code of Military Justice, a murder conviction could carry a penalty of death. But there are some procedural differences between the two systems.

General Casey sent a directive to commanders to keep soldiers informed about the case and urge them to avoid a rush to judgment, saying he wanted to avoid a backlash against Muslims. There are 1,977 soldiers in the active-duty Army who identify themselves as Muslims, of a total of 553,000 active-duty troops, according to the service.

The emerging portrait of Major Hasan is of a man who came from an immigrant family defined by upward mobility. His parents came to the United States in the early 1960s from a village on the West Bank and settled first in Northern Virginia before moving to Roanoke to open a series of small businesses, including restaurants and a store.

Paul M. Holt III, a private investigator who went to high school with Major Hasan, called his parents “salt of the earth,” saying, “If you were hungry and didn’t have enough money, they’d let you come back later and pay for it.”

But like many others, Mr. Holt described Major Hasan as having few friends and being quiet to the point of introversion. “He wasn’t very personable,” Mr. Holt said. “I can’t imagine him sitting and listening to people’s problems.”

After graduating with a degree in biochemistry fromVirginia Tech, in nearby Blacksburg, he was commissioned as an officer and sent to medical school at the Uniformed Services University of the Health Sciences in Bethesda, where he graduated in 2003. He did his internship and residency at Walter Reed Army Medical Center in Washington before entering a two-year fellowship that gave him a master’s degree in public health and trained him in disaster psychiatry.

Two students in the fellowship program said Major Hasan had sat alone in the front of the class and rarely socialized with other students, other than to debate the wars in Iraq and Afghanistan. He made clear he strongly opposed both, the former students said.

Major Hasan also told family members that he had experienced anti-Muslim harassment in the Army and had tried for several years to be discharged. But the Army, which had paid for his education and was in great need of psychiatrists, refused, family members said.

“Just recently, we received phone calls from the family, saying that Nidal was unhappy because he had big problems at work,” said Mohammed Mohammed, a cousin of Major Hasan’s from Ramallah on the West Bank.

In an interview at their home there, Mr. Mohammed and another cousin, Mohammed Hasan, said they had kept up with Major Hasan’s situation through his brother Anas, a lawyer who recently moved to Ramallah.

Major Hasan’s family in Virginia said he had been afraid of deploying because he had heard all about the horrors of war from returning soldiers. But the cousins in Ramallah offered another reason: he was soon to be wed and did not want to leave his spouse.

The cousins said they learned of the shooting in the early hours of the morning on Friday when relatives phoned them, then saw the news on television. “I was dumbfounded,” said Mohammed Hasan. “I tried to listen for somebody else’s name. But unfortunately it was my cousin.”

The cousins described how Major Hasan and his two brothers had turned to religion after the death of their parents, their father in 1998 and their mother 2001.

“They became very religious after their mother died,” Mohammed Hasan said. “They were very observant. They prayed a lot.”

But he denied that they were religious to the point of fanaticism. “Their religion had nothing to do with politics,” he said.

In an interview on NBC’s “Today” show, Lt. Gen. Robert W. Cone, the Fort Hood commander, was asked about reports that before opening fire, Major Hasan had yelled “Allahu akbar!” — “God is great!” General Cone said soldiers at the scene had reported “similar” accounts.

Muslims who attended mosques with Major Hasan in Virginia, Maryland and Texas said they had never heard him express extremist views about politics or religion. And though openly opposed to the wars, he did not express anti-American sentiments, they said.

At the Muslim Community Center in Silver Spring, Md., Major Hasan was a regular at Friday prayers who also dropped by on occasion to help on a homeless program. Sabir Rahman, past president of the mosque’s board, called him “a very gentle person” who “never expressed strong words about anybody.”

At the Dar al-Hijrah mosque in Falls Church, Va., where Major Hasan’s family often prayed and he occasionally went, the imam, Sheik Shaker Elsayed, recalled that his greatest interest seemed to be in finding a wife.

Yahya Hendi, a part-time chaplain at the National Naval Medical Center in Bethesda, said Major Hasan had once praised him for giving a sermon opposing extremism. “He felt that Muslims needed to speak about peace and love,” Imam Hendi said.

In Killeen, a neighbor of Major Hasan’s, Willie Bell, got a call from him on Wednesday night. Mr. Bell had given Major Hasan his wireless password, and now the major was asking him to turn on his Internet system. “He said, ‘Nice knowing you, friend,’ ” Mr. Bell said. “ ‘I’ll be moving.’ ”

On Thursday morning, Major Hasan, without wearing his traditional Muslim cap, left his apartment complex in his car, Mr. Bell said. The F.B.I. confiscated Mr. Bell’s laptop, he said.

Clifford Krauss reported from Killeen, Tex., and James Dao from Washington. Michael Brick, Gretel C. Kovach, Serge Kovaleski and Ray Rivera contributed reporting from Killeen; David Johnston and Charlie Savage from Washington; Damien Cave from Miami; Erica Goode from New York; Ashley Southall from Silver Spring, Md.; Theo Emery from Roanoke, Va.; Wendy Grossman from Falls Church, Va.; and Catrina Stewart from Ramallah, West Bank.