WASHINGTON - The US military on Friday faced fraught questions about the recruitment, safety and role of Muslims in the armed services after a shooting massacre blamed on a devout Muslim officer.

About 3,500 service members in the 1.4 million-strong US armed forces call themselves Muslim, and the man suspected of slaying 13 people at a Texas army base on Thursday, Major Nidal Malik Hasan, is one of them.

It remained unclear what led the army psychiatrist to allegedly open fire on his fellow soldiers, but an emerging portrait of Hasan suggested his Islamic faith and sensitivities may have played a decisive role.

His aunt said he had faced anti-Muslim harassment on the job and wanted a discharge, while a co-worker told Fox News he had expressed anger over the US war in Iraq and spoke of the need for Muslims to "stand up and fight against the aggressor."

As a criminal investigation got under way, Islamic-American groups called for calm amid fears of a possible backlash while some rightwing commentators spoke of the threat of a Muslim "fifth column" infiltrating the army.

The military leadership, which prides itself on a diverse force and a code of tolerance, said there were no immediate signs of an angry reaction against Muslims serving in the armed forces.

"I would say I fear it," General George Casey, US Army chief of staff, told reporters at Fort Hood. Casey said he had told leaders not to "rush to judgment or speculate until the investigation comes out. "I do worry slightly about a potential backlash and we have to be all concerned about that."

The shooting also raised concerns about how to vet soldiers for possible signs of extremism, with security officials warning more often of late about the danger of "homegrown terrorism." Hasan's case serves as a recruiter's ultimate nightmare scenario, even though the military takes pains to check potential recruits on a range of criteria.

A spokesman for the top US military officer, Admiral Mike Mullen, said it was too early to consider changes in recruiting or related policies. He said the military was not "taking steps at this time to change any of our recruiting, retention or ascension type procedures," Captain John Kirby told AFP.

"There is a lot more that we don't know about what spurred this man to do what he did than there is about what we do know," he said. Muslims have been serving in the American military for years and they were as likely to seek discharges over US wars in Islamic countries as they were to enlist to demonstrate their patriotism, said Bill Galvin, a Vietnam veteran and a counseling coordinator at the Center on Conscience and War. "Certainly there have been Muslims in the military for ... quite a while, and many of them serve honorably and well by military standards," Galvin said, who serves on the board of the GI Rights Hotline.

His organization had heard from at least six Muslims who wanted to leave the military as conscientious objectors, he said. Commentators on the right called the shooting a "terrorist act," accusing senior officers of failing to see the danger posed by militants inside the military. "When an extremist plans and executes a murderous plot against our unarmed soldiers to protest our efforts to counter Islamist fanatics, it's an act of terror. Period," wrote Ralph Peters in the New York Post.

Hasan's Muslim identity grabbed media attention and overshadowed the nature of his work, a psychiatrist listening to soldiers distraught and depressed by their experiences in the cauldron of war. Some experts said investigators needed to explore how his work may have affected his outlook and possibly caused him to snap.

His aunt, Noel Hasan, said his sessions with traumatized soldiers had taken a toll. She told the Washington Post that he recounted one man so badly burned "that his face had nearly melted ... He told us how upsetting it was to him." Muslim and Arab groups in the United States were anxious to emphasize that one incident should not obscure the loyal military service of many Muslims and Arabs.

"Thousands of Arab Americans and American Muslims serve honorably everyday in all four branches of the US military and in the National Guard," including those now deployed in Afghanistan and Iraq, the Arab-American Institute said.