JOAN LOWY, Associated Press

WASHINGTON (AP) — The government has been refusing for more than three years to release a study that found air traffic controllers’ work schedules often lead to chronic fatigue, making them less alert and endangering the safety of the national air traffic system.

Federal Aviation Administration officials ignored repeated requests by The Associated Press for a copy of a report on the study and haven’t filled a Freedom of Information Act request for the document. However, the AP was able to obtain a draft of the final report dated Dec. 1, 2011.

The study found that nearly 2 in 10 controllers had committed a significant error in the previous year — such as bringing planes too close together — and over half the controllers attributed the errors to fatigue.