Officials said the review would focus on the exact number of Muslims in the U.S. military, which has encouraged such enrollment. The Defense Department reported 3,409 Muslims on active military duty as of April 2008, but officials said the number could be at least three times higher.

"We believe there are many more Muslims who when recruited did not list their religion," an official said. "Some of these people simply wanted to avoid harassment; others might have had a sinister agenda."

Over the last decade, the military has intensified its recruitment of Arabic-, Farsi- and Pashtun-speaking soldiers for the wars in Afghanistan and Iraq, Middle East Newsline reported.

Officials said the Pentagon has been receiving reports of Muslim soldiers who expressed opposition to the U.S. military campaigns in Afghanistan and Iraq. They said the opposition was encouraged by Islamic clerics as well as Muslim officers such as Hasan, who warned Muslims against harming co-religionists.

In 2003, U.S. Army Sgt. Hasan Akbar, said to have opposed the U.S. invasion of Iraq, killed two officers and injured 14 others in a grenade attack. Akbar, a convert to Islam, was sentenced to death.

Officials said the U.S. military has sought to shield Muslims from retaliation in Afghanistan and Iraq. In many cases, they said, Muslim soldiers were ordered to use fake family names to prevent reprisals against their families abroad.

Some in Congress have called for clear guidelines on allowing soldiers to express political views in the military. They said Hasan's pro-jihad views were tolerated by officers concerned over charges of discrimination.

"I want to say very quickly we don't know enough to say now, but there are very, very strong warning signs here that Dr. Hasan had become an Islamist extremist and, therefore, that this was a terrorist act," Sen. Joseph Liberman, a Connecticut independent and chairman of the Senate Homeland Security and Governmental Affairs Committee, said.

Lieberman said his committee would investigate the Hasan shooting, particularly the motive for the attack. He said a focus would be whether the U.S. Army ignored warning signs that Hasan was heading for an attack.

Officials acknowledged that Hasan underwent an investigation in April 2009 on suspicion that he had adopted Al Qaida doctrine of holy war against the West. They said Hasan was suspected of trying to contact Al Qaida via the Internet.

"I am intending to begin a congressional investigation of my homeland security committee into what were the motives, what were the motives of Hasan in carrying out this brutal mass murder and to ask whether the army missed warning signs that should have led them to essentially discharge him," Lieberman said.

"If Hasan was showing signs, saying to people that he had become an Islamist extremist, the U.S. Army has to have zero tolerance. He should have been gone."