Staff assistants to the Sept. 11 commission are planning a trip to the National Archives to retrieve their notes on a U.S. military unit's information that four of the Sept. 11, 2001 hijackers were inside the United States a year before the attacks, FOX News has learned.

Defense Department documents show that the information, developed by a classified defense intelligence unit dubbed "Able Danger," wasn't handed over to the FBI because of concerns about pursuing information on foreigners admitted to the country for permanent residence.

A source familiar with the Sept. 11 commission told FOX News on Wednesday that the aides who still have security clearances are looking for a memo about a briefing given to four staff members by defense intelligence officials during an overseas trip to Afghanistan, Pakistan and Saudi Arabia in Fall 2003.

The date of the archives expedition is not yet known. The National Archives are located just outside of Washington, D.C.

The activity comes after Lee Hamilton (search), co-chairman of the now-disbanded commission, said he wanted to know whether defense intelligence officials knew of the Al Qaeda-linked attackers' activity but failed to tell law enforcement.

In an interview with FOX News, Hamilton said there should be a comprehensive review by Congress and the Pentagon into the claims. He said this potentially cruicial information could change the way history sees Sept. 11, 2001.

Members of the commission are reviewing claims that more than a year before the 2001 attacks defense intelligence officials had identified ringleader Mohammed Atta (search) and three other hijackers, and that they were already inside the United States. A statement could come by the end of the week

"The Sept. 11 commission (search) did not learn of any U.S. government knowledge prior to 9/11 of surveillance of Mohammed Atta or of his cell," said Hamilton, a former Democratic congressman from Indiana. "Had we learned of it obviously it would've been a major focus of our investigation."

Hamilton's remarks Tuesday followed findings by Rep. Curt Weldon (search), R-Pa., vice chairman of the House Armed Services and Homeland Security committees, that made front-page news.

In June, Weldon displayed charts on the floor of the U.S. Senate showing that Able Danger identified the suspected terrorists in 1999. The unit repeatedly asked for the information to be forwarded to the FBI but apparently to no avail. Various news outlets picked up on the story this week.

Weldon told FOX News on Wednesday that staff members of the Sept. 11 commission were briefed at least once by officials on Able Danger, but that he does not believe the message was sent to the panel members themselves. He also said some phone calls made by military officials with Able Danger to the commission staff went unreturned.

"Why weren't they briefed? Was there some deliberate attempt at the staff level of the 9/11 commission to steer the commissioners away from Able Danger because of where it might lead?" Weldon asked. "Why was there no mention of Able Danger?"

Pentagon spokesman Bryan Whitman said the Sept. 11 commission looked into the matter during its investigation of government missteps leading to the attacks and chose not to include it in the final report.

A group of Sept. 11 widows called the September 11th Advocates issued a statement Wednesday saying they were "horrified" to learn that further possible evidence exists, and they are disappointed the Sept. 11 commission report is "incomplete and illusory."

"The revelation of this information demands answers that are forthcoming, clear and concise," the statement said. "The Sept. 11 attacks could have and should have been prevented."

Sept. 11 Staffers Investigate

Hamilton confirmed that commission staff members learned of Able Danger during a meeting with military personnel in October 2003 in Afghanistan, but that the staff members do not recall learning of a connection between Able Danger and any of the four terrorists now mentioned. He also said no mention made of Atta.

It was "inconceivable" that staffers would have missed such a reference, Hamilton told FOX News.

According to the source who spoke with FOX News, none of the staffers believe they were ever told specifically about Atta having been identified by defense intelligence before the 2001 attacks.

But after the October 2003 trip, the commission staff members pursued Able Danger further and asked the Pentagon to produce documents related to the unit, which they were, FOX News has also learned.

Still, Defense Secretary Donald Rumsfeld said this week that he was unaware of the intelligence until the latest reports surfaced.

The source said three key questions need to be answered: Did defense intelligence identify Atta before the attacks? If so, was this information ever passed onto the commission? Was Rumsfeld ever briefed by his predecessor, Bill Cohen, on the unit and its findings?

"If the Sept. 11 commission staff made a mistake and missed this information, we will be the first to admit it. That is why we are going back to the archives to check our work, but at this time, staff do not believe this information was ever made available to them," the source said.

The commission's report on the terrorist attacks, released last year, traced government mistakes that allowed the hijackers to succeed. Among the problems the commission cited was a lack of coordination between intelligence agencies.

With the Sept. 11 commission disbanded for a year under provisions of the legislation that created it, some of the panel's members have said congressional committees should investigate Weldon's assertions.

"I can tell you right now, there are investigations going on right now… trying to get answers," Weldon told FOX News on Wednesday.

A 'Series of Mysteries'

According to Weldon, Able Danger identified Atta, Marwan al-Shehhi (search), Khalid al-Mihdar (search) and Nawaf al-Hazmi (search) as members of a cell Able Danger code-named "Brooklyn" because of some loose connections to New York City.

Weldon said that in September 2000, the unit recommended on three separate occasions that its information on the hijackers be given to the FBI "so they could bring that cell in and take out the terrorists." However, Weldon said Pentagon lawyers rejected the recommendation, arguing that Atta and the others were in the country legally so information on them could not be shared with law enforcement.

"Lawyers within the administration — and we're talking about the Clinton administration, not the Bush administration — said 'you can't do it,'" and put post-its over Atta's face, Weldon said. "They said they were concerned about the political fallout that occurred after Waco ... and the Branch Davidians."

Defense Department documents show that the Able Danger team was set up in 1999 to identify potential Al Qaeda operatives for U.S. Special Operations Command. Information provided to the team by the Army's Information Dominance Center (search) pointed to a possible Al Qaeda cell in Brooklyn.

However, because of concerns about pursuing information on "U.S. persons" — a legal term that includes U.S. citizens as well as foreigners admitted to the country for permanent residence — Special Operations Command didn't give the Army information to the FBI. It is unclear whether the Army provided the information to anyone else.

The command instead turned its focus to overseas threats.

The documents provided no information on whether the team identified anyone connected to the Sept. 11 attacks on New York City and Washington that killed nearly 3,000 people.

The prohibition against sharing intelligence on "U.S. persons" should not have applied since they were in the country on visas and did not have permanent resident status, said Weldon.

Able Danger was largely using open-source information that was available on the Internet and in other public media, he added.

Click here for FOXNews.com's story on open-source information and the War on Terror.

Bob Graham, former chairman of the Senate Intelligence Committee, told FOX News on Wednesday that Congress not only needs to investigate the Able Danger claims but also needs to investigate other related issues, such as how German intelligence agencies alerted the United States that members of the Hamburg Al Qaeda cell were coming to America.

What also needs to be investigated further, Graham said, is how two Sept. 11 terrorists were living in a building in San Diego where their landlord was actually an FBI informant. The FBI is not saying what the informant told the agency about those terrorists, he said.

"I anticipate [Congress] will be getting on to their job with a great deal of commitment and expertise in this area. I hope it doesn't end with this one instance of why we didn't know about Atta," said Graham, author of "Intelligence Matters."

"There's not just one mystery here — there's a series of mysteries about why we didn't learn about this plot early enough to break it up," he added.

FOXNews' Catherine Herridge and Liza Porteus and The Associated Press contributed to this report.