Air traffic controllers who direct planes in and out of Chicago's O'Hare International Airport have too little time off between shifts, a factor that contributes to fatigue and could be a safety threat, according to a federal audit released Thursday.

The 21-page report by the U.S. Department of Transportation criticized the Federal Aviation Administration for not acting sooner to alleviate air controller fatigue at three of the country's busiest air traffic facilities and recommended immediate changes.

The audit showed most controllers at the three facilities located in the Chicago area have had fewer than 10 hours rest between some shifts, progressively earlier start times on consecutive shifts and increased overtime hours.

"This type of work schedule offers minimal opportunity for sleep when the time required for commuting, eating and other necessary daily activities is taken into account," the report said.

The report said the FAA has not acted on earlier National Transportation Safety Board recommendations on controller fatigue and that the agency "does not consistently address human factors issues, such as fatigue and situational awareness" when it investigates operational errors.

O'Hare has had a series of near misses in recent years that were blamed on air traffic controller error. In June, the FAA said two American Airlines planes came close to each other while landing at O'Hare, though both planes landed safely and no injuries were reported.

The transportation department's report laid out recommendations, including increasing rest periods, rotating controllers through less demanding positions during each shift and providing fatigue awareness training.

FAA spokeswoman Elizabeth Isham Cory said the agency disputes some of the audit, but has already made changes in the past year. For example, the FAA added training on fatigue awareness in May 2008. In light of the audit, the FAA said it would re-evaluate staffing and look at increasing rest periods between shifts.

"Certainly, fatigue is a concern of ours. This is something that we're watching very, very closely," Cory said Thursday. "We have made changes already and we continue to make changes."

The audit, which was sent to FAA earlier this week, was conducted between January 2008 and February at a tower at O'Hare and facilities in the Chicago suburbs of Elgin and Aurora. A spokeswoman for U.S. Sen. Dick Durbin, D-Ill., said he requested the audit after hearing controllers' concerns.

Air traffic controllers said they felt "vindicated" by the audit's findings.

"We have expressed our concerns about controller fatigue for several years, only to have them fall on deaf ears at the FAA, which has ignored NTSB requests to meet with us and work on fatigue issues in a spirit of collaboration," said Patrick Forrey, president of the National Air Traffic Controllers Association, a union which represents about 15,000 air traffic controllers.