Law enforcement officials have subpoenaed records from a number of universities and colleges around the country in an attempt to seal what they believe is a major security loophole: the United States' open-door policy toward foreign students.

Several people detained in recent days and questioned about possible ties to the Sept. 11 hijackings and attacks on the World Trade Center and Pentagon were in the country on student visas even though they only briefly or never attended school, investigators say.

The reports have raised alarm among lawmakers, who are scrambling to fix what they believe is a highly flawed program.

"I know this isn't politically correct, but what has happened indicates, I think, serious caution is a prudent thing on our part," said Sen. Dianne Feinstein. "Our country is a sieve. These visas are being misused and the time has come to do something about it," she said.

Hani Hanjour, believed to be one of the hijackers on the flight that hit the Pentagon, was in the country on a student visa allowing him to study English at Holy Names College in Oakland, Calif. Hanjour, however, had never enrolled.

Hussein al Attas, a man detained in Oklahoma in connection with the attacks, entered the United States on a student visa that had expired. A second man arrested in San Diego was also in the country on an expired student visa, and a third detainee, Makkaram Ali, claimed to be in the States attending a city college but had dropped out.

Habib Moussaoui, who authorities say is an aide to Usama bin Laden — the exiled Saudi billionaire U.S. officials have identified as the mastermind behind the attacks — also entered the U.S. on a student visa after mailing money to a flying school in Oklahoma he found on the Internet.

Nineteen suspects in the trade center and Pentagon bombings received flight training at 10 U.S. vocational schools, and 44 additional suspects now wanted by the FBI received at least some flight training.

Ira Mehlman, a spokesman for the anti-immigration Federation For American Immigration Reform, said students entering the country have virtually free reign once they arrive.

"We don't know if they are attending the universities they are supposed to be attending. We don't even know where in the country they are," Mehlman said. "And we don't know if they leave the USA when they're supposed to."

This week, at least three schools in the California State University system turned over student records to the FBI, and a federal grand jury in Boston subpoenaed records from the University of Rhode Island. The University of California at Berkeley, Auburn University in Alabama and Utah's Weber University were also among about 25 institutions that authorities investigating the Sept. 11 attacks have contacted.

Student visas, issued by U.S. Consulates in the students' home country, have long been identified as a dangerous and unchecked loophole in immigration policy. Critics say they are too easy to get and that once in the nation, foreign students are not monitored. The government does not track whether they are actually attending school, or whether they remain in the country after the visa expires.

Currently, more than 500,000 foreign students attend American colleges on student visas, 10,000 from nations with ties to terrorism — Iran, Iraq, Libya, and Sudan among them.

Angela Kelly, a spokeswoman for the National Immigration Forum, said the U.S. consular employees who issue student visas are often young and untrained State Department personnel without access to FBI and INS databases and so-called "look-out" lists of suspected terrorists and criminals.

"The consulate offices should be the first line of defense. They need people trained and experienced in detecting document fraud and access to intelligence," Kelly said. "That's not the case right now. The information is not always available to the consulate or it is not up to date."

State Department spokesmen Richard Boucher said the department has begun reviewing student visa procedures.

"Obviously, we're looking at this whole process and how to make it safer, how to make it better, but that review, the look, the process of reviewing, is just beginning," Boucher said.

The Sept. 11 terrorist attacks have touched off a review of overall immigration policy, with several members of the House and Senate proposing legislation to reform the system.

Feinstein, D-Calif., wants to close the U.S. borders to new foreign students for six months to give immigration authorities the time to conduct background checks and enact a tracking system for foreign students once they are in the country.

The legislation, which she said she would propose in coming weeks, would give the INS $32.3 million for a computerized monitoring system to track foreign students.

Congress authorized such a database after the 1993 World Trade Center bombing (several of the suspects tied to that attack were also in the country on student visas) but the program was thwarted by political opposition and is now only a small pilot project involving about 25 southeast colleges.

The universities contacted this week would not disclose what information authorities were seeking. At the University of Rhode Island, spokesperson Linda Acciardo said the school believed the subpoena was a "general request."

"We have no reason to believe it's connected to the individuals who hijacked the planes," she said.

David Ward, the president of the American Council on Education — whose members include most of America's accredited colleges and universities — opposes Feinstein's proposed suspension of student visas, but supports the tracking system.

"Our view would be that since student visas are only two percent of the total [number of immigrants], it isn't really solving the security problem," Ward said. "People can come as visitors and in many other ways," he said.

The University of Southern California, New York University and the University of Wisconsin-Madison have the highest number of foreign students, according to the ACE.

Fox News' William La Jeunesse and The Associated Press contributed to this report.