Federal Aviation Administration officials say an error from air traffic controllers caused a military jet and a commuter plane to fly close to each other over northern Illinois.
But officials say the two planes were never in danger of colliding during the Sunday incident.
The FAA says the planes were a United Express jet headed to Lexington, Kentucky, from Chicago's O'Hare International Airport and a C-130 military jet.
They got within about three miles of each other.
The FAA says an experienced controller was supervising a controller in training when they both instructed the commuter plane to climb to 23,000 feet, rather than its requested final altitude of 21,000 feet. The C-130 was at 22,000 feet.
Both controllers have been taken off-duty and sent for retraining.