Two FedEx jets got too close to each other on takeoff from the Memphis airport last week because of confusion over flight numbers for the planes, a Federal Aviation Administration spokeswoman said Thursday.

The incident, which the FAA classified as serious, air traffic control error, remained under investigation.

The air traffic controllers union blamed the incident and other recent mistakes by controllers at Memphis on short staffing and forced overtime.

Kathleen Bergen, an FAA spokeswoman in Atlanta, disagreed. “We have never found fatigue to be an issue in any of the errors that have occurred at Memphis,” Bergen said.

On Feb. 21, two FedEx jets flying in the same direction on takeoff came within 200 feet vertically and three-fourths of a mile horizontally to each other, Bergen said. Big jets in the Memphis area are supposed to keep a distance of at least 1,000 feet vertically and 3 miles horizontally.

“Those airplanes were extremely close together and that’s just seconds away from the two of them hitting,” said Ron Carpenter, local president of the National Air Traffic Controllers Association.

Bergen said the flights had similar numbers _ 527 and 257 _ that caused confusion sending one plane on a wrong turn shortly after they took off at the same time on separate runways.

The incident began with the pilot of one plane getting on the wrong radio frequency “and accepting instructions intended for another aircraft,” Bergen said. FAA rules require pilots to read back flight instructions and for controllers to make sure they were properly received.

“The controller didn’t recognize the error on read-back,” Bergen said.

Bergen said the controller was decertified and will be retrained. The pilot’s error was still under investigation.

FedEx declined comment, noting the investigation was ongoing.