Each year, about one million foreign students enter the country on visas to attend American schools. But the Department of Homeland Security admits it doesn’t know the whereabouts of 6,000 of these visitors.

Among the participants of the September 11, 2001, terrorist attacks was Hani Hanjour, who flew Flight 77 into the Pentagon after entering the United States on a student visa. Since that fateful day, 26 student visa holders have been arrested in the U.S. on charges related to terrorism, according to U.S. Senator Charles Coburn (R-Oklahoma).

The 9/11 Commission subsequently put forth recommendations for tightening up the student visa program. But in spite of that, immigration officials have continued to allow educational institutions to accept foreign students “even if the schools lack accreditation, state certification, or any obvious measure of academic rigor,” according to ABC News’ Brian Ross and Matthew Mosk.

The government-approved list includes more than 9,000 schools, including those specializing in beauty, massage, golf, acupuncture and flight training. It is each school’s responsibility to always know the whereabouts of their student visa-holders and to notify federal officials of chronic absences. Visa-holder Hanjour vanished from his classes prior to flying a jet into the Pentagon, as did Eyad Ismoil, the student visa-holder who subsequently drove an explosives-filled van into the World Trade Center garage in February 1993.

Coburn is among the members of Congress who have expressed concern about weaknesses in the educational visa tracking system, particularly since he sees the selling of visas—not teaching—as the real goal of many for-profit colleges.

“We know we have a lot of non-accredited universities that are using this system to bring people in, collect money, and not educate them at all,” Coburn told ABC News. “To me, it’s a mess.”

Immigration and Customs Enforcement (ICE) says that 58,000 students remained in the country after their visas expired over the past year. Those missing 6,000 are now being sought out by immigration agents.

“My greatest concern is that they could be doing anything,” Peter Edge, ICE’s top official for investigating visa violators, told ABC News. “Some of them could be here to do us harm. We really have a lot more work to do.”

9/11 Commission co-chair Thomas Kean agrees that gaps in the student visa program could prove to be disastrous for U.S. security. “It's been pointed out over and over and over again and the fact that nothing has been done about it yet... it’s a very dangerous thing for all of us,” he told ABC News. “The fact that there's been no action on this is very bothersome.”

- Danny Biederman, Noel Brinkerhoff

To Learn More:

Lost in America: Visa Program Struggles to Track Missing Foreign Students (by Brian Ross and Matthew Mosk, ABC News)

More than 800,000 Foreign Students in U.S.; Most Study Business, Science and Engineering (by Steve Straehley, AllGov)