Washington (CNN) -- U.S. authorities have kept about 350 people with "suspected ties to terrorism" off U.S.-bound planes since January 2010, officials said Monday.
Prior to the Christmas Day 2009 bombing attempt by Umar Farouk AbdulMutallab, the alleged "Underwear Bomber," the U.S. vetted passengers of U.S.-bound flights against a very select no-fly list before flights departed.
However, in most cases, the government did not vet passengers against more comprehensive terror watch lists until the flights had departed overseas airports and were en route to the U.S. Passengers discovered to be on watch lists were intercepted at airports, questioned and placed on return flights if necessary.
But the December 2009 incident on board Northwest flight 253 brought to light the gap in aviation security. Since then, the U.S. tightened security protocols.
In that instance, AbdulMutallab's father had contacted authorities at the U.S. embassy in Abuja, Nigeria, with concerns that his son had become radicalized and was planning something. But those concerns were never relayed to keepers of the large database of suspected individuals. AbdulMutallab was permitted to fly on the jetliner and unsuccessfully tried to ignite an underwear bomb, authorities said. He has since pleaded not guilty to six federal terrorism charges.
The incident prompted a new U.S. Customs and Border Protection Pre-Departure targeting process, introduced in January 2010, which identifies passengers on the terror watch lists and denies them boarding on U.S.-bound flights.
About 350 people have been denied boarding on U.S.-bound flights because they had suspected ties to terrorism, Customs and Border Protection officials confirmed Monday. Officials declined to reveal the factors that put people on the watch list, but gave as examples people who have financial or familial ties to terrorists.
The number of denials was first reported Monday by the Associated Press.
All together, 2,859 inadmissible people have been kept off the U.S.-bound flights under the new program, CBP spokesman Michael Friel said. Those who don't have suspected terror ties are denied boarding for a variety of reasons, including having revoked, lost or stolen passports (both foreign and U.S.), or being denied under electronic travel authorization.
A Department of Homeland Security official, speaking on background Monday because he was not authorized to discuss the program, said it is impossible to say how many people would not have flown to the U.S. in previous years if the new protocol had been in effect.
Currently, about 12,000 people, including 1,000 Americans, are on the broader no-fly list, which consists of people who are considered a threat to aviation.
Another approximately 18,000 are on the selectee list of people who face additional screening at airports but are allowed to fly.
And 450,000, including about 6,000 American citizens, are on the broader Terrorist Screening Database, described as a list of people who are, or could be, a threat to national security due to terrorist ties. They too face extra screening but are allowed to fly.
CNN Senior Producer Carol Cratty contributed to this story