Radical Imam Anwar al-Awlaki and Hasan attended the controversial Dar al-Hijrah mosque in Great Falls, Virginia, in 2001 at the same time as two of the Sep 11 terrorists, The Sunday Telegraph has learnt.
His mother's funeral was held there in May that year.
Anwar al-Awlaki is an American-born Yemeni scholar who was banned from addressing a meeting in London by video link in Aug because he is accused of supporting attacks on British troops and backing terrorist organizations.
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.
As investigators look at Hasan's motives and mindset, his attendance at the mosque could be an important piece of the jigsaw.
Relatives said that the death of Hasan's parents, in 1998 and 2001, turned him more devout.
His failed search for a wife also seemed to haunt Hasan.