As the passage of time zooms in on the fourth anniversary of the Sept. 11 attacks, stories about that horrific day and the days immediately afterward are destined to become more numerous. They will span recollections about that tragic day, the latest news about security steps that have been implemented as a result of the attacks plus unfinished business, such as the failure to capture al-Qaida leader Osama bin Laden.

Amid all of that could come troubling revelations and allegations, such as one that surfaced earlier this month. An Army intelligence officer said his unit was blocked in 2000 and 2001 from giving the FBI information about a U.S.-based terrorist cell that included Mohamed Atta, the future leader of the Sept. 11 attacks.

The Army officer is Lt. Col. Anthony Shaffer, who says a small intelligence unit called "Able Danger" had identified Atta and three of the other future Sept. 11 hijackers as al-Qaida members by mid-2000. According to Shaffer, military lawyers stopped the unit from sharing its information with the FBI.

Able Danger's claims were not included in the official report of the commission that investigated the Sept. 11 attacks.

Shaffer's allegation is in line with the opinion expressed by U.S. Rep. Curt Weldon, R-Pa., that the Sept. 11 commission did not adequately investigate the claim that four of the hijackers had been identified more than a year prior to the attacks. Weldon is vice chairman of the House Armed Services and Homeland Security committees - a position that puts him in an advantageous position for confidently making such an assessment.

A closer look at what Shaffer has said is necessary, despite the fact that the special commission has completed its work.

The panel's conclusions are not beyond question, just like aspects of the Warren Commission's investigation of the assassination of President John F. Kennedy still are questioned nearly 42 years after the president's death. It would not be a surprise if other troubling allegations about 9/11 would surface in coming years, considering the lack of communication between agencies that pervaded the nation's intelligence network prior to 9/11.

If accurate, Shaffer's claims should be the basis for extreme uneasiness on the part of those who scuttled the sharing of the information in question in the year prior to the attacks. There's also basis for uneasiness on the part of those who actually took possession of the data that Able Danger turned over to the 9/11 commission.

It could be that the American people will never receive a full explanation tied to Shaffer's allegations, but disclosures such as his must not be ignored.

Regardless of how embarrassing, acknowledging pre-Sept. 11 errors could help to avert other terrorist assaults against this country.