The FBI is set to investigate possible links between the gunman who killed 13 US soldiers at Fort Hood and a radical preacher in contact with at least one of the September 11 hijackers.

Major Nidal Malik Hasan, an army psychiatrist, went on the rampage at the military base in Texas last Thursday.

It has emerged that Major Hasan's family attended the Dar al Hijrah Islamic Centre in Falls Church, Virginia, and that the funeral of his mother, Hanan, was held there on May 31 2001.

At that time the imam of the mosque was Anwar al-Awlaki, a Muslim radical who saw Islam and America as enemies. Interviewed by the FBI after the September 11 terror attacks, Mr Awlaki admitted meeting hijacker Nawaf al-Hazmi several times in San Diego before he moved to Virginia in early 2001. Al-Hazmi was at the time living with Khalid al-Mihdhar, another hijacker. The two Saudi Arabians were on the American Airlines flight that crashed into the Pentagon.

Al-Hazmi and a third hijacker, Hani Hanjour, who piloted the Pentagon crash jet, attended the Dar al Hijrah mosque several times in early April 2001, although Mr Aulaqi has denied meeting them then.

Last night Senator Joe Lieberman called for an investigation into whether the army missed signs that Major Hasan had been turning towards Islamic extremism.

Students on a 2007-2008 master’s programme at a military college revealed yesterday that they had complained to faculty about Major Hasan’s alleged anti-American views.They included him giving a presentation that justified suicide bombing and telling classmates that Islamic law trumped the US Constitution.

“If Hasan was showing signs, saying to people that he had become an Islamist extremist, the US Army has to have zero tolerance,” Senator Lieberman, an independent from Connecticut, told Fox News. “He should have been gone.”

Mr Awlaki, who was born in the US to Yemeni parents, left America in 2002, eventually traveling to his parents' homeland.

He was suspected of links with extremism even before 9/11, being investigated by the FBI in 1999 and 2000 for contacts with a possible procurement agent for Osama bin Laden. During this investigation, the FBI learned that Mr Awlaki knew people involved in raising money for Hamas, a Palestinian group on the US State Department’s terrorist list.

A spokesman for the mosque downplayed possible links. Imam Johari Abdul-Malik, outreach director at Dar al Hijrah, said he did not know whether Major Hasan ever attended the mosque but confirmed that the Hasan family participated in services there. Mr Abdul-Malik said the Hasans were not leaders at the mosque and their attendance was utterly normal.

The Falls Church mosque is one of the largest on the East Coast, and thousands of worshippers attend prayers and services there every week. Mr Abdul-Malik said that it was a mistake for people to conflate regular attendance at a mosque with extremism.

Many Muslims prayed at the mosque multiple times a day, he said. “It’s part of family life. It’s like going out for ice cream after dinner.”

Faizul Khan, former imam of the Muslim Community Center in nearby Silver Spring, Maryland, where Major Hasan also worshipped, said he was not aware that the gunman had attended services at Dar al Hijrah but said it would not be unusual for him to attend more than one mosque concurrently.

Mr Khan said he did not recall Major Hasan mentioning having been taught or preached to by Mr Awlaki.

General George Casey, the US Army Chief of Staff, warned against jumping too quickly inclusions, saying that it was important for the country not to get caught up in speculation about Major Hasan’s Muslim faith. He revealed that he has instructed his commanders to be on the lookout for anti-Muslim reaction to the killings at the Texas post.

Focusing on the Islamic roots of the suspected gunman could “heighten the backlash" against all Muslims in the military, he said in interviews on TV networks ABC and CNN, adding that diversity in the military “gives us strength”.

General Casey said that evidence to this point shows that Major Hasan acted alone, ruling out theories that he was part of an Islamic sleeper cell.

Major Hasan was shot in the torso by police at the end of his rampage, and remains in critical but stable condition at an Army hospital in San Antonio. He was now able to breathe unaided, doctors said.

Sixteen victims remain in hospital with gunshot wounds, seven of them in intensive care.

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