The Transportation Department said Thursday that it thinks inexperience may be to blame.
A government investigator said the Federal Aviation Administration may be putting too many trainee controllers at some of the nation's busiest airports.
The agency plans to hire 11,000 new controllers by 2019 to make up for a wave of retirements, Inspector General Calvin Scovel said. Many of the more than 15,000 controllers now working for the FAA were hired following President Ronald Reagan's firing of striking controllers three decades ago, and are now eligible for retirement.
Scovel said that as of March, 25 percent of controllers were in training compared to 15 percent in 2004.
However the FAA said the increase may be the result of a new program that encourages controllers to report mistakes without fear of reprisal.
The Associated Press contributed to this report.