WASHINGTON — U.S. citizens who are considered a terrorist threat and banned from flying on passenger airplanes can nonetheless learn to fly without hindrance, a loophole that emerged during a congressional hearing Wednesday into security lapses at the nation's 935 accredited flight schools.

"I'm shocked to hear that someone on the no-fly list can be approved for flight lessons," said Rep. Mike Rogers, R-Ala., a member of the House Homeland Security Committee. "It is mind-blowing."

U.S. citizens are screened against terrorism databases 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.

Kerwin Wilson, the Transportation Security Administration (TSA) official who oversees the flight-school screening program, said he did not know whether an American on the no-fly list has undergone flight training in the U.S. in the past 10 years.

"Keep in mind, the way the program is set up, there's layered security in place," Wilson said, adding that once someone receives a flight certificate, he or she is screened against other criminal and terrorism databases regularly. Wilson also cautioned that putting U.S. citizens through these additional security checks could cost more money.

After the Sept. 11 attacks, when al-Qaida terrorists who had attended flight schools in Florida, Arizona and Minnesota hijacked and crashed four commercial jetliners, 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 in recent years about "homegrown" terrorists launching attacks on U.S. soil.

U.S. flight schools are generally less expensive and more rigorous than those in other countries, and often enroll a large number of foreign students each year. About 30 percent of students enrolled in flight classes in the U.S. are foreign nationals.

An audit of the flight-school screening program by the U.S. Government Accountability Office found that existing measures are falling short. Investigators found that some foreigners had completed flight training without a full background check, and that some flight-school students were in the country illegally.

Homeland-security officials launched an investigation in 2010 after a Boston-area aviation school was found to have been training illegal immigrants to fly airplanes. Investigators have identified 30 people who may be in the country illegally and successfully attended flight schools. They are under investigation for immigration violations.

Material from The Associated Press is included in this report.