The federal commission that probed the Sept. 11, 2001, terror attacks was told twice about "Able Danger," a military intelligence unit that had identified Mohamed Atta and other hijackers a year before the attacks, a congressman close to the investigation said Wednesday.

Rep. Curt Weldon (search), R-Pa., a champion of integrated intelligence-sharing among U.S. agencies, wrote to the former chairman and vice-chairman of the Sept. 11 commission late Wednesday, telling them that their staff had received two briefings on the military intelligence unit — once in October 2003 and again in July 2004.

Weldon said he was upset by suggestions earlier Wednesday by 9/11 panel members that it had been not been given critical information on Able Danger's capabilities and findings.

"The impetus for this letter is my extreme disappointment in the recent, and false, claim of the 9/11 commission staff that the commission was never given access to any information on Able Danger," Weldon wrote to former Chairman Gov. Thomas Kean (search) and Vice-Chairman Rep. Lee Hamilton (search). "The 9/11 commission staff received not one but two briefings on Able Danger from former team members, yet did not pursue the matter.

"The commission's refusal to investigate Able Danger after being notified of its existence, and its recent efforts to feign ignorance of the project while blaming others for supposedly withholding information on it, brings shame on the commissioners, and is evocative of the worst tendencies in the federal government that the commission worked to expose," Weldon added.

On Wednesday, a source familiar with the Sept. 11 commission — formally known as the National Commission on Terrorist Attacks Upon the United States (search) — told FOX News that aides who still had security clearances had gone back to the National Archives outside Washington, D.C., to review notes on Atta and any information the U.S. government had on him and his terror cell before the Sept. 11 attacks.

The source acknowledged that the aides were 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 the fall of 2003.

Staffers apparently did not recall being told of the Able Danger information at that meeting and wanted to double-check their records.

Former commission spokesman Al Felzenberg told The New York Times in Thursday editions that Atta was mentioned to panel investigators during at least one meeting with a military officer. That briefing came in July 2004, less than two weeks before the commission's final report was issued to the public.

Felzenberg said the information about Atta was considered suspect because it didn't jibe with many other findings. For example, the intelligence officer said Atta was in the United States in late 1999, but travel records confirmed that he did not enter the country until late 2000.

"He wasn't brushed off," Felzenberg told The Times about the military officer's briefing. "I'm not aware of anybody being brushed off. The information that he provided us did not mesh with other conclusions that we were drawing."

But Weldon said that argument was not good enough.

"The 9/11 commission took a very high-profile role in critiquing intelligence agencies that refused to listen to outside information. The commissioners very publicly expressed their disapproval of agencies and departments that would not entertain ideas that did not originate in-house," Weldon wrote in his letter Wednesday night.

"Therefore it is no small irony," Weldon pointed out, "that the commission would in the end prove to be guilty of the very same offense when information of potentially critical importance was brought to its attention."

On Thursday, Weldon told FOX News that the military official, who was under cover when he was in Afghanistan for the October 2003 briefing, is certain he told the staffers about Atta at that time.

The military intelligence officer who attended that meeting with staffers "kept notes of that meeting and will testify under oath that he not only told" the staffers about Able Danger's mission, but about Atta.

Hamilton, a former Democratic congressman from Indiana, told FOX News on Wednesday that if Atta's name had been mentioned in the October 2003 briefing, it would have jumped out at staffers.

He said that the commission did not include the claims by Able Danger in the definitive report of the events leading up to Sept. 11 because it had no "information that the United States government had under surveillance or had any knowledge of Mohamed Atta prior to the attacks.

"It could be a very crucial incident in terms of the lead-up to 9/11. It could reveal flaws in the intelligence sharing or the lack of intelligence that we have not yet focused on," Hamilton said of the military's tracking of Atta and its inability to get domestic intelligence agencies to follow up.

Hamilton told FOX News that the commission team would get to the bottom of the confusion over what the United States knew about Atta and whether it played into the commission's investigation.

"I think the 9/11 commission's obligation at this point is to review our records very, very carefully and make very soon — we hope within the next few days — a complete statement about what happened during our investigation," Hamilton said.

Weldon said that he personally knows five members of the commission and is not attacking the integrity of any of them. He said he discussed the matter with two commissioners who told him they were never briefed about Able Danger.

"I have to ask why. I would hope there was not a deliberate attempt by someone on the 9/11 commission staff to keep this information" from the commissioners, Weldon said, adding "I find no fault right now with the commissioners."

A commission spokesman told FOX News that the panel expected to issue a statement before the end of the week.

Among the most critical facts to be determined, if the information about Atta did exist in 2000, would be who then blocked the intelligence from going to the FBI, which could have tracked down the terror cell.

"Team members believed that the Atta cell in Brooklyn should be subject to closer scrutiny, but somewhere along the food chain of administration bureaucrats and lawyers, a decision was made in late 2000 against passing the information to the FBI," Weldon wrote.

"Fear of tarnishing the commission's legacy cannot be allowed to override the truth. The American people are counting on you not to 'go native' by succumbing to the very temptations your commission was assembled to indict," he added.