An investigation by the Fox News Specials Unit revealed that the commission investigating the Sept. 11 attacks was likely never told by the FBI about an arrest warrant filed and then dropped against American-born radical Muslim cleric Anwar al-Awlaki – the spiritual guide to at least two of the hijackers.

Now, lawmakers on Capitol Hill want details on the handling of the Awlaki case – particularly why information on the possible arrest was never provided to the 9/11 Commission. The New Mexico-born cleric, now working as a recruiter for Al Qaeda in Yemen, is believed to have been a mentor for 9/11 hijackers during his time as an imam at mosques around San Diego and Washington, D.C.

Despite his possible role in the attacks and inconsistencies in his passport, a warrant for al-Awlaki’s arrest was dropped on Oct. 10, the same day he returned to the U.S. after a seven-month stay in Yemen.

Two senior investigators with the 9/11 Commission told Fox News that they were actively considering whether Awlaki was part of a pre- 9/11 support cell in the U.S. for the hijackers. Asked if documents on the decision by federal authorities to drop the charges should have been shared, one of the investigators said yes, adding the decision to pull the warrant on the same day Awlaki returned in October 2002 demanded further investigation.

There is no mention of the Awlaki arrest warrant in the 9/11 Commission report itself. There is a lengthy footnote about the cleric which has one reference to an FBI internal memo that was circulated Oct. 8, 2002 – just two days before the cleric’s return. Fox News sought those documents through a Freedom of Information Act request in 2010. Twenty-seven pages were produced for Fox News’ investigative team, but nearly all the text was redacted – citing an executive order – that the material should remain secret for national security reasons.

On Tuesday, FBI Director Robert Mueller was on Capitol Hill, where he faced tough questions over the documents from Rep. Frank Wolf, R- Va.

“I wrote you concerning the Fox News report about the bureau's October 2002 decision to release Anwar Awlaki from custody upon his return to the U.S. despite an outstanding warrant for his arrest at that time, “ Wolf said, adding that commission investigators had also voiced their concern to Fox News. “Could you discuss why the information was not shared with the 9/11 Commission and provide the committee with a detailed accounting in the appropriate setting?”

Mueller said he remembered looking at the warrant, which originated out of Denver, Colo., in the summer of 2002 a “couple of years ago,” adding “there was not much more -- not much more and certainly at this time to be done.” The FBI director did not elaborate.

“If this was not provided to the 9/11 Commission, it was certainly not by intent,” Mueller said. “It may well have been not in a file, one of the files that we provided to that 9/11 Commission. But I think you will find the 9/11 Commission was appreciative of all the work that we had done to provide them, you know, just about everything that we had in our files.”

Mueller said he would get back to the congressman who first wrote to the FBI in the summer of 2010 about Awlaki’s mysterious return to the U.S. after the Fox News Special: The American Terrorist was broadcast.

“I would be happy to look into the matter. I can tell you that we tried to provide everything that we had that would be responsive to the interests of the 9/11 Commission at the time.”

The Justice Department has maintained that Awlaki’s re-entry to the U.S. was a string of coincidences. The Department told Fox News in a statement last May that there was not enough evidence to keep the warrant against Awlaki active. Two current and former investigators familiar with the case have disputed that claim.

New details about the cleric’s American life and his connections to the 9/11 hijackers will be part of an upcoming Fox News special.

Catherine Herridge is an award-winning Chief Intelligence correspondent for FOX News Channel (FNC) based in Washington, D.C. She covers intelligence, the Justice Department and the Department of Homeland Security. Herridge joined FNC in 1996 as a London-based correspondent.