A glaring loophole in counter terrorism emerged during a congressional hearing Wednesday into security lapses at the nation’s 935 accredited flight schools – U.S. citizens who are considered a terror threat and banned from flying on passenger airplanes can attend flight lessons without hindrance.

According to an article from The Los Angeles Times, U.S. citizens are screened against terrorism database only after flight training, when they apply for a pilot’s license. More than 550 U.S. citizens are on the no-fly list – a database kept by the FBI’s Terrorist Screening Center.

The Al Qaeda terrorists who intentionally crashed jetliners into the World Trade Center, the Pentagon and a field in Pennsylvania on September 11, 2001, had attended flight schools in Florida, Arizona and Minnesota. Afterward, security checks were added for people coming to the United States to enroll in flight schools, but those checks were never extended to U.S. citizens, despite growing concerns about “homegrown” terrorists, the LA Times reports.

U.S. flight schools are generally considered less expensive and more rigorous than those in other countries, and often enroll a large number of foreign students annually. About 30 percent of flight school students are foreign nationals, the article says.

An audit of flight school screening programs by the U.S. Government Accountability Office found that existing measures are falling short. Investigators found that some foreigners had completed their training without a full background check, and that some were in the U.S. illegally, according to the article. Thus far, investigators have identified 30 people who may have entered the country illegally and successfully attended flight schools.