Following Sept. 11, Capt. Abd Al-Rasheed Muhammad, imam of Walter Reed Army Medical Center in Washington, D.C., asked the North American Islamic Jurisprudence Council if it is permissible for Muslim troops in the U.S. military to fight other Muslims in the war against terrorism.
The council referred the matter to Muslim clerics abroad, who issued a fatwa permitting U.S. Muslims to fight if there was "no alternative."
The chain of command of the U.S. military did not remain intact for long. The Middle East Media Research Institute (www.memri.org) reports that the Arab clerics reconsidered and withdrew permission on Oct. 30, issuing a new fatwa prohibiting U.S. Muslim troops from participating in U.S. attacks on Muslim forces.
from Nov. 20, 2001 http://www.newsmax.com/commentmax/articles/Paul_Craig_Roberts.shtml the article may have changed and you will therefore have to "hunt" for the original on that web site.
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As soon as the U.S. geared up for the war against the Taliban regime in Afghanistan, Muslim military personnel in the American armed forces began to deal with the question of the religious permissibility of their participation in battle. Army Chaplain Capt. Abd Al-Rasheed Muhammad, the Imam of Walter Reed Army Medical Center in Washington, D.C. sent an inquiry on the matter to the North American Fiqh (Islamic Jurisprudence) Council, which in turn referred the matter to clerics in the Arab world. The clerics issued a Fatwa permitting Muslim soldiers to take part in the fighting if there was no alternative, and the council delivered the ruling to Capt. Muhammad. But on October 30, the editor of the Arabic London daily Al-Sharq Al-Awsat reported that the clerics who signed this Fatwa had changed their minds and abrogated their previous Fatwa with a new one prohibiting participation of Muslim soldiers in the war in Afghanistan. Capt. Abd Al-Rasheed Muhammad estimates there are approximately 15,000 Moslems in thge U.S. Armed Forces.
http://www.memri.org/ Nov. 6, 2001 use that web site's search engine to find the report.