In the never-ending effort to find someone — anyone — to blame for Sept. 11, the latest scrum involves claims by an ambitious congressman about a secret Defense Department unit pursuing a pilot program in data mining.

Data mining is a technique in which huge databases are fed into powerful computers that sift them looking for links. It's a technology that holds vast promise, but its main usefulness to date seems to be giving mortgage lenders the ability to find out how much you still owe on your house.

The Defense data-mining program, established in 1998, was called Able Danger. It focused on identifying potential al-Qaida terrorists. Two weeks ago, Curt Weldon, a melodramatic Republican congressman from Pennsylvania, introduced it to the world via Fox News and The New York Times. The story has changed in its details almost daily since, as asserted high-flying facts have fallen to ground, but the gist has remained the same:

Sometime before Sept. 11, 2001 — the dates vary but lately have settled around the first few months of 2000 — the Able Danger computers came up with the name Mohamed Atta as a potential terrorist associated with an al-Qaida cell in Brooklyn, N.Y. Atta, remembers Weldon and a pair of military men he has dragged before the cameras and notebooks, was one of four 9/11 hijackers identified by the computers more than a year before the attacks. They can't produce records, and the Pentagon denies it, but it's what they remember seeing, hearing and being told.

I know nothing about Able Danger other than what I've read, so I can't speak with authority on what the program uncovered about Atta, or when. But, having spent the better part of the last four years investigating Atta's life, I can speak to what is otherwise known about him and his whereabouts.

Atta's academic, immigration, credit, transit and telephone records provide a fairly complete account from the time he left his native Egypt in autumn 1992 to his death. This includes the period during which Able Danger is said to have identified him as a terrorist in the United States. The story those records, and corroborating interviews, tell is that Atta was not in the United States and made almost no contact with the United States until June 2000.

In November 1999, Atta and three friends traveled from Germany — via Istanbul and Karachi — to Afghanistan, where they intended to receive military training before going to fight the infidels in Chechnya. They were, instead, recruited into al-Qaida and assigned the Sept. 11 mission. Atta returned to Hamburg in late February, and the next month he made what is thought to be his first contact with someone in the United States. He e-mailed dozens of flight schools inquiring about commercial pilot training for "a small group of Arab men." He also e-mailed a friend from Egypt who was studying at a Florida university and asked about visa requirements. In May, he applied for a visa from the U.S. Embassy in Berlin. Six weeks later he landed in Newark, N.J.

It is hard to see how computers could have named Atta as a member of an American cell before he got here. Some have argued that perhaps Able Danger mined data that included flight records of young Arab men traveling to Pakistan. Even if it did, it probably would not have found Atta. He was listed on airline flight manifests as Mohamed el-Amir, not Atta. His full name was Mohamed Mohamed el-Amir Awad el-Sayed Atta. El-Amir is how Atta was known to friends at school, to the banks that issued his credit cards and to the immigration service in Germany. It's the name on his high school and college diplomas.

Even if Able Danger somehow produced a name, "Mohamed Atta," that might not mean much. Variations of "Mohamed" are overwhelmingly the most common name in the Muslim world. The likelihood of mistaken identity is enormous.

But there is another possibility. Over the last four years I have interviewed dozens of people who swore they saw Atta somewhere he wasn't. This includes an assortment of waiters, students, flight instructors, taxi drivers and, more dramatically, two women who each claim to have been married to Atta, this despite the fact that they were never in the same city at the same time he was.

How could it be that so many people remember that they knew Atta, that they saw him or his name, when all the facts argue otherwise? I think Atta entered an American psyche desperate for a name and face and an explanation. He came complete with what has become one of the iconic images of 9/11 — his Florida DMV mug shot, an image so memorable, so powerful and perfect for the moment that it allowed people to see in it whatever they needed to see. I think people subsequently, subconsciously placed that face where it made sense to them.

Whatever the resolution of the Able Danger imbroglio, there were plenty of missed opportunities on the road to 9/11. German law enforcement knew in mid-1999 that Atta and Marwan al-Shehhi, another Sept. 11 hijacker, were acquaintances of an al-Qaida recruiter. This information was passed on to the CIA. The name of a third hijacker, Ziad Jarrah, was given to U.S. intelligence agencies in early 2000 when he was interrogated at length as he passed through customs in the United Arab Emirates en route from Afghanistan to Germany. He told Emiratis he was going to the United States to become a pilot. The Emiratis say they passed this information to the Americans.

More famously, the CIA tracked two known al-Qaida operatives through eight CIA stations from the Middle East to Malaysia, then somehow didn't notice as they walked onto a jetway and a plane bound for Los Angeles. We don't need to invent intelligence failures; we need to grapple with those that we already have.

Terry McDermott is a Los Angeles Times staff writer and the author of "Perfect Soldiers: The Hijackers — Who They Were and Why They Did It" (HarperCollins, 2005).