Government aviation inspectors allowed unsafe aircraft to operate in U.S. commercial flight operations, a whistle-blower investigation has concluded.

The Federal Aviation Administration’s own review confirmed the allegations, according to the Office of Special Counsel, the agency charged with investigating complaints from whistle-blowers.

“Safety inspectors at the Federal Aviation Administration have improperly approved aircraft for commercial operations without first reviewing critical safety information that in some cases would have prohibited their operation,” the Special Counsel said in a statement on Wednesday.

The planes, mostly with 30 or fewer seats, were being used by smaller carriers that mainly operate charters. In one example, some charter operators were allowed to fly with exit doors that weren’t easily accessible during an emergency, a violation of regulations, according to the Special Counsel.

After the allegations were brought to FAA’s attention, the agency last year agreed with the findings and created a task force to evaluate records for over 11,000 aircraft. The FAA “identified several corrective actions as a result of its investigation” and expects to complete them by Sept. 30, the agency said in an emailed statement.

The investigation also uncovered evidence that aircraft were allowed to operate in the U.S. with expired registrations. The FAA said most of the registration violations were inadvertent and didn’t put travelers at risk.

“It appears that the whistle-blower identified serious problems in the FAA’s oversight,” the Special Counsel wrote in a letter to the White House reporting the findings.