Thousands of students who came into America with student visas to study do not have their locations accounted for in the Department of Homeland Security.

"My greatest concern is that they could be doing anything," Peter Edge, the U.S. Immigration and Customs Enforcement official who oversees investigations into visa violators, told ABC News. "Some of them could be here to do us harm."

The missing students may have once had valid visas, but the time on their document has expired and they haven't left the country, which makes it harder for the Department of Homeland Security to track them. After the 9/11 terrorist attacks, these rules were supposed to tighten because one of the plane hijackers was in the country on a student visa, according to ABC.

Since the 9/11 terror attacks in 2001, 26 student visa holders were arrested on terror charges, Sen. Tom Coburn, R-Okla., told ABC.

Some students used student visas as a gateway to illegal immigration because their visas were valid for work while they were in school, and for up to 29 months post-graduation, immigration agents told investigators, according to Washington Times.

After students stop attending classes, it's more difficult for schools and the government to keep track of what the student is doing and if the student leaves when their time is up.