Last month, two senators asked Airlines for America, a trade group representing American Airlines, Delta Air Lines, United Airlines and others for answers after more than 2,700 Memorial Day weekend flights were canceled.

The group said in a previously unreported letter dated Friday that "airlines are aggressively pursuing several options to align schedules with workforce availability" but added "the FAA must also work to ensure that the air traffic control system is capable of meeting demand."

(Reporting by David Shepardson)