The preacher at the time was Anwar al-Awlaki, an American-born Yemeni scholar who was banned from addressing a meeting in London by video link in August because he is accused of supporting attacks on British troops and backing terrorist organisations.

Hasan's eyes "lit up" when he mentioned his deep respect for al-Awlaki's teachings, according to a fellow Muslim officer at the Fort Hood base in Texas, the scene of Thursday's horrific shooting spree.

Charles Allen, a former under-secretary for intelligence at the Department of Homeland Security, has described al-Awlaki, who now lives in Yemen, as an "al-Qaeda supporter, and former spiritual leader to three of the September 11 hijackers... who targets US Muslims with radical online lectures encouraging terrorist attacks from his new home in Yemen".