Lectures by Anwar al-Awlaki were found in the Iqra bookshop in Leeds, where the July 7 bombers held meetings. His lectures were also found among the material seized from a jihadist recruiter, Aabid Hussain Khan, from Bradford, West Yorkshire, who groomed Britain's youngest terrorist, schoolboy Hammaad Munshi, then 15.
Al-Awlaki , 38, a US citizen, fled from the country to live in Britain after it was disclosed that three of the September 11 hijackers had worshipped at two different mosques where he preached.
He moved to Yemen in 2004, where he was arrested and then went underground, but he has been invited to give video lectures, most recently at East London Mosque in January, and his videos are still on sale in Britain.
Major Hasan, an army psychiatrist, worshipped at a mosque in Virginia where al-Awlak was imam at the same time as two Sept 11 hijackers.
Hasan, 39, has been charged with 13 murders. He shouted "allahu akbar" ("God is great") and shot dead unarmed soldiers awaiting vaccinations before or after deployment. He is lying in a military hospital bed, paralysed from the waist down, after civilian police officers shot him to end the rampage.
It has since emerged that US intelligence monitored email correspondence during the past two years between the officer and the imam.
Hasan asked advice on jihad and whether it was permissible to kill innocent people in a suicide attack.
Al-Awlaki's most influential lecture, Constants on the Path to Jihad, is based on a 2005 book by Yousef al-Ayyiri, the founder of al-Qaeda in Saudi Arabia.
According to Evan Kohlmann, a US terrorism consultant, the sermon was "a 'virtual bible' for lone-wolf Muslim extremists".
Earlier this year, al-Awlaki also published 44 Ways to Support Jihad, a practical point-by-point guide to pursuing or supporting holy war.
"Constants spells out al-Qaeda's home-grown terror strategy," said Mr Kohlmann.
"Al-Awlaki didn't just translate it, he applied its teachings for a Western audience. So when Ayyiri stated 'jihad is everywhere', al-Awlaki explained that meant jihad was in Europe too.
This is a very clever individual who understands the power of the Internet to radicalise people. Had al-Awlaki not translated Constants, then most of the Western Muslims who were inspired by it would never have even heard of it."
Al-Awlaki 's lectures were also found on a CD belonging to Mohammed Atif Siddique, 22, from Glasgow, an aspiring suicide bomber, who was found guilty of possessing bomb-making instructions.