The US military 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 right-wing commentators spoke of the threat of a Muslim "fifth column" infiltrating the army.
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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 judgement 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 counselling coordinator at the Centre on Conscience and War.