The terrorist watch lists at the center of what President Obama called the botched handling of intelligence on a plot to blow up an airliner on Christmas Day have been in the spotlight since the 9/11 terrorist attacks.

Before the hijackings of four jets in 2001, intelligence and law enforcement agencies kept different lists, and information sharing among them was spotty. The 9/11 Commission found that the poor coordination helped allow the attackers to enter the country, and it strongly recommended improvements.

Significant enhancements have been made since then. However, lawmakers and security experts began calling this week for revisions that would make it harder for suspected terrorists to smuggle a bomb aboard a jet — but they could make the screening process at airports more cumbersome.

"It was just one database failure after another," said Clark Kent Ervin, former inspector general at the Homeland Security Department.

Weeks before Umar Farouk Abdulmutallab, 23, allegedly touched off explosives on a Northwest Airlines flight from Amsterdam to Detroit, his father raised suspicions about him to U.S. officials in Nigeria.

That prompted Abdulmutallab's name to be entered into the Terrorist Identities Datamart Environment (TIDE) database, a catch-all repository for even the most raw intelligence. The TIDE is administered by the National Counterterrorism Center, an arm of the nation's intelligence agencies, and contains about 560,000 entries, the agency says.

His name was never elevated into the nation's consolidated terrorist watch list, said Chad Kolton, spokesman for the Terrorist Screening Center, a consortium of agencies managed by the FBI. The center oversees the centralized watch list, which has 400,000 names.

To be included on that list, the government has concluded there is a "reasonable suspicion" the person is linked to terrorism, Kolton said. Names are shared across the government, but the list plays no role in whether a person may board a flight, Kolton said.

Other, much smaller watch lists overseen by the same agency are used to screen potential terrorists who want to fly on U.S. carriers. One list, containing fewer than 4,000 names, bars individuals from flying. Another with fewer than 16,000 triggers additional scrutiny, such as pat-downs or tests for explosives, but it allows people to fly.

In both cases, the vast majority of names on the lists are not U.S. citizens or legal residents, Kolton said. Though the criteria for inclusion on these lists are secret, Kolton said they include people considered threats to aviation.

Philip Zelikow, former director of the 9/11 Commission, said making additional improvements in the system may be necessary, but it will be difficult and costly.

Adding more names to the list of people who receive additional screening could lengthen security lines and require hiring more screeners, he said.

Still, the vulnerabilities exposed by the Christmas plot should be plugged, he said.

"We may end up being grateful that we got such a big scare without having any funerals," Zelikow said. "The system needed another jolt. People were getting a little complacent."

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Alan Levin reports for USA Today.