As word spread that a gunman had opened fire at Fort Hood leaving a trail of carnage, a chilling realization swept across the U.S. Muslim community: He has an Islamic name.

From a professor who just testified in Congress, to a White House adviser appearing before a Jewish group and a former Marine driving home from work, Muslims across the country were shocked, angry and afraid that the attack would erode efforts to erase anti-Islamic stereotypes.

Many Islamic leaders said the Fort Hood tragedy that left 13 dead and 30 wounded including the alleged gunman, Maj. Nidal Malik Hasan, could likely pose the sternest test for U.S. Muslims since the Sept. 11 terrorist attacks.

"A lot of us work very hard for this country, to make America a better place," said Muqtedar Khan, a progressive Muslim scholar who has just given Congressional testimony on U.S. foreign policy in Afghanistan before Thursday's attack. "And this one nut like Maj. Hasan comes along and in one crazy episode of a few seconds he undermines these years and years of hard work we are doing to make American Muslims part of the mainstream in the community."

Hasan, an Army psychiatrist, is a Muslim who attended his former mosque daily and had an "Allah is Love" bumper sticker on his car. Soldiers reported Friday that the shooter shouted "Allahu Akbar!" -- Arabic for "God is great!" -- during the rampage.

Other troubling details also emerged, including reports that authorities suspect Hasan posted online messages about suicide bombers and violence, was struggling with a pending deployment to Afghanistan and was being harassed in the Army for being a Muslim.

While a motive remains unclear, the confirmation of Hasan's faith alone prompted major Muslim groups and mosques to issue statements condemning the killings as contrary to Islam and praising the service of the many Muslim Americans in the U.S. military.

Of immediate concern was security at mosques Friday, Islam's main day of communal prayer.

In Washington, Chicago and elsewhere, mosques asked police for extra patrols. In Garden Grove, Calif., officers stood watch outside a mosque as a precaution.

Muslim leaders warned people to be vigilant and avoid exposing themselves unnecessarily -- including walking alone, said Hussam Ayloush, director of the Council on American-Islamic Relations in Southern California.

"This is one of those moments where we have to sit and pray that most Americans will come out stronger, more united, and more tolerant," said Ayloush, adding that Muslim organizations have received dozens of death threats and hate e-mail.

At the Muslim Community Center in Silver Spring, Md., which Hasan attended before moving to Fort Hood, Imam Mohamed Abdullahi urged worshippers Friday to tell their non-Muslim neighbors that Islam was not responsible for the deaths. He also advised them to keep their tempers in check.

"Whenever we hear the name turns out to be Arabic or Muslim we feel a double shock" about such incidents. "And then we worry about backlash," said Imam Mostafa Al-Qazwini of the Islamic Educational Center of Orange County in Costa Mesa, Calif.

U.S. Rep. Andre Carson, an Indiana Democrat who is one of two Muslims serving in Congress, cautioned against focusing on the alleged shooter's religion and instead said the discussion should be about mental health issues.

"This is no way a reflection of Islam any more than Timothy McVeigh's actions are a reflection of Christianity," said Carson, who supervised an anti-terrorism unit in Indiana's Department of Homeland Security and comes from a family of Marines.

Eboo Patel, the executive director of Chicago-based Interfaith Youth Core, had just spoken at a Union of Reform Judaism conference in Toronto on Thursday night when a rabbi told him: "The guy had a Muslim name."

"I had just spoken from the tradition of Islam ... on the importance of interfaith cooperation and building Muslim-Jewish bridges," said Patel, who sits on a White House faith-based advisory board. "I wish that was viewed as reflective of Islam instead of a deranged lunatic who was acting only in the tradition of deranged lunacy, not in the tradition of any faith."

But other Muslims were weary of what has become a routine: a Muslim does something unspeakable, and Islamic organizations issue statements condemning it.

"Truth be told, we're getting a little exhausted because we've done this to death," said Robert Salaam of Maryland, a former Marine who converted to Islam shortly after the 9-11 attacks and now blogs and hosts a radio show on Muslim affairs. "We're apologizing for people we don't know."

Still, driving home from work listening to the news Thursday, Salaam thought: "God, I hope it's not a Muslim."

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Associated Press writers David Dishneau in Silver Spring, Md.; Amy Taxin in Tustin, Calif.; Sophia Tareen in Chicago; Jeff Karoub in Detroit; and Peter Prengaman in Atlanta contributed to this report.