The Federal Aviation Administration is looking into whether Delta Air Lines violated FAA rules about promoting safety at a time when President Donald Trump’s pick to lead the agency was in charge of Delta’s flight operations.

The FAA investigation grew out of allegations by a Delta pilot that the airline retaliated against her for raising safety concerns. The Associated Press obtained a copy of an FAA letter sent to the pilot’s attorney detailing the investigation. The FAA declined to comment on the probe.

Trump’s nominee, Stephen Dickson, is under growing criticism from Senate Democrats over his initial failure to disclose his involvement in the case of the whistle-blowing pilot, who was grounded a few weeks after she raised safety issues to Dickson and other Delta executives.