WASHINGTON - The killings of 13 people at Fort Hood, Texas, allegedly by an Army psychiatrist who also was a Muslim, set off a rancorous debate that once again spotlighted the fear among Muslims in America that they would be collectively found guilty for the actions of one man.

Vitriolic exchanges filled Internet sites devoted to military affairs late last week, with some posters arguing that Muslims should be barred from the armed services.

News reporters deluged the Silver Spring, Md., mosque where the Fort Hood shooting suspect once worshipped, demanding to know what the Quran, Islam's holy book, had to say about such events. One asked if the suspect, Army Maj. Nidal Malik Hasan, who was born in Virginia and has lived his whole life in the United States, spoke with an "accent."

Anita Husseini, who also worships at the Muslim Community Center, said she didn't know Hasan, but she knew that what he was accused of doing would affect her life and those of others.

"My heart cried last night," said Husseini, a hairdresser. "Every time the Muslims try to get up, something goes boom and pushes us back. What a crazy person decides does not define me or Islam."

The mosque's chairman, Arshad Qureshi, said: "They're trying so hard to pin this on Islam. They're working so hard to make it about religion."

U.S. military officials said the motive behind Thursday's shooting remained unclear.

Investigators seized the suspect's computer after news reports said someone named Nidal Hasan had posted messages comparing suicide-bombing missions to Japanese kamikaze pilots.

Lt. Gen. Robert W. Cone, the base commander at Fort Hood, said witnesses to Thursday's mayhem reported that Hasan had shouted "Allahu akbar," or "God is great" in Arabic, as he opened fire on clusters of soldiers awaiting medical examinations and other processes.

The phrase is a traditional Muslim invocation.

Cone and others, however, turned away questions about Hasan's religion, and Cone said there was no evidence that Hasan was part of a wider plot.

There is no doubt within the military hierarchy about the loyalty of Muslim service members, said Maj. Gen. Kevin Bergner, a Pentagon spokesman. The military, he said, will take steps to make sure "everyone is treated with dignity and respect."

Posters to Facebook and participants in chat rooms and popular military sites Friday were less circumspect, revealing a bitterness that Muslims say they have often felt since the terrorist attacks of Sept. 11, 2001.

One Facebook page was titled "Against muslims in military! . . . or in presidency" - a reference to the false claims that President Obama is a Muslim.

Muslims make up less than 0.3 percent of America's active-duty military forces. Of the roughly 548,000 soldiers in the U.S. Army, 2,500 say they are Muslims, 1,500 of them on active duty.

By comparison, 105,000 claim Roman Catholicism as their religion, and 99,000 say they are Baptists. More than 1,800 soldiers say they are Jewish, surpassed by the nearly 2,500 who identify themselves as atheists.

More than 101,000 list no religious affiliation. That was the case with Hasan, according to Pentagon officials, though interviews at the Silver Spring mosque make clear he was an observant Muslim who prayed daily - and often in uniform.

Mona Ayad, the administrative assistant at the center, said Hasan would come to prayer quite often, volunteer at the mosque, contribute money for the poor as Islam requires, and answer phones. He stopped coming over the summer, apparently when he was transferred to Fort Hood.

Imam Mohamed Abdullahi Sheikh Mohamed, the mosque's chief cleric, said he knew Hasan from his frequent appearances at the mosque and knew he was a military doctor. But he said Hasan never brought up his work with the U.S. military.

Hasan's religious affiliation was known to his military colleagues and might have caused tensions. His family said he was harassed for his faith, and the Washington Post quoted his aunt Friday as saying Hasan sought legal help in an effort to get out of the Army.

One Army chaplain spoke on the condition of anonymity out of fear that an interview would turn Thursday's shooting into a religious issue.

The chaplain said that some Muslims are conflicted about honoring their duty while fighting other Muslims in Iraq and Afghanistan. In those cases, Muslim soldiers usually prefer talking to a Muslim chaplain, he said. They also more often turn to Muslim chaplains when they feel harassed in the military.

The chaplain said many of the soldiers he talked to felt betrayed mainly because Hasan is a fellow soldier.

"This is not a Muslim issue. It is a soldier issue," he said. "It is a punch in the gut."

In his Friday sermon, Mohamed paid his condolences to the families of those killed. He quoted a Quran verse that says taking one innocent life is like killing humankind.

Over loudspeakers, Mohamed said, "I ask the media people not to relate all the time everything to Islam."