WASHINGTON (AP) — The U.S. government has prevented more than 350 people suspected of ties to al-Qaida and other terrorist groups from boarding U.S.-bound commercial flights since the end of 2009, The Associated Press has learned.

The tighter security rules — imposed after the attempted bombing of an airliner on Dec. 25, 2009 — reveal a security threat that persisted for more than seven years after the Sept. 11, 2001, terrorist attacks.

Until then, even as commercial passengers were forced to remove their shoes, limit the amount of shampoo in their carry-on luggage and endure pat downs, hundreds of foreigners with known or suspected ties to terrorism passed through security and successfully flew to the United States each year, U.S. officials told the AP. The government said these foreigners typically told Customs officers they were flying to the U.S. for legitimate reasons such as vacations or business.

Security practices changed after an admitted al-Qaida operative from Nigeria was accused of trying to blow himself up on a flight to Detroit on Christmas 2009. Until then, airlines only kept passengers off U.S.-bound planes if they were on the no-fly list, a list of people considered a threat to aviation.

Now before an international flight leaves for the U.S., the government checks passengers against a larger watch list that includes al-Qaida financiers and people who attended training camps but are not considered threats to planes. The government was checking this list before, but only after the flight was en route to the United States. If someone on the flight was on the watch list, the person would be questioned and likely refused to enter the country after the plane landed.

“As terrorists keep adapting and changing their approach, so must we,” Sen. Jay Rockefeller, a Democrat, 0told the AP. During a Senate hearing shortly after the attempted Christmas attack, Rockefeller raised concerns about divisions among the different watch lists.

Hundreds of people linked to al-Qaida, Hamas, Lashkar-e-Taiba and other militant groups have been kept off airplanes under the new rules. They include what U.S. officials described as a member of a terrorist organization who received weapons training, recruited others, fought against American troops and had a ticket to fly to the U.S. Another traveler prevented from boarding a U.S.-bound flight was a member of a terrorist organization whom intelligence officials believe had purchased equipment for terrorism.

A third case, in January, involved a Jordanian man booked from Amman, Jordan, to Chicago, who was considered a threat to national security, according to a law enforcement official. The State Department had already revoked his visa. He was on the terrorist watch list but not the no-fly list. He was not considered a threat to aviation.

After U.S. Customs and Border Protection officers performed the now-routine check, the man was kept off the flight. Before the change, he would have arrived in Chicago, where he would have likely been stopped at customs, questioned and sent home.

The law enforcement official and other U.S. officials insisted on anonymity to discuss sensitive security issues. They would not provide the names of the people suspected of terror ties or some key details about the cases for security reasons.

“We’ve gotten better with our techniques and applying them predeparture, ensuring we’re looking at as broad a section of potential risk as possible,” said Kevin McAleenan, deputy assistant commissioner of field operations at Customs and Border Protection, or CBP.

CBP said the gap in U.S. security practices wasn’t obvious until after the attempted Christmas attack. Officials were prepared to question the accused bomber when he landed in Detroit — but that turned out to be too late.

“We had the skill set, the systems and the techniques, and we needed to move backwards in time,” McAleenan said.

Since the Sept. 11 attacks, the terror watch list and its derivative, the no-fly list, became some of the government’s best-known counterterrorism tools. They also became some of the most criticized, as innocent travelers were inconvenienced when they were mistaken for terrorism suspects. Outrage forced the government to pare the lists, which airlines checked before allowing people to fly.

After the attempted Christmas attack, the intelligence community took a closer look at the names on the terror watch list and set new standards for adding names. The watch list and no-fly list are constantly reviewed, and names are added and removed each day. There are about 30,000 people on the no-fly list and a companion list for people who must receive extra screening at airports, a counterterrorism official told the AP.

The more expansive terror watch list includes about 450,000 names of people the U.S. intelligence community believes are, or could be, a threat to national security because of terrorist ties. Some of the people on the watch list are still being investigated, and there is not enough information for the government to arrest them.

The new policy has not turned the 450,000-person terror watch list into the no-fly list. Simply being on the terror watch list does not mean a person won’t be allowed to enter the U.S., McAleenan said. When CBP reviews passenger lists and matches someone on the terror watch list, CBP will review information available on the person before it recommends to the airline whether the person can board the plane, McAleenan said. In most cases, if CBP recommends against allowing the passenger to board, it’s because the person would be turned away upon arrival inside the United States due to security concerns.

Most people on the watch list are foreigners. About 6,000 are U.S. citizens.

American citizens who are not considered threats to aviation but are on the terror watch list cannot automatically be prevented from flying to the U.S.

Customs officers will likely question a U.S. citizen who is on the terror watch list when he or she comes into the country. But without grounds for arrest, the officers must let them arrive. This also applies to a U.S. citizen who is on the no-fly list but who walks or drives back into the U.S. through land-border crossings.