Two airliners landed at Reagan National Airport near Washington without control tower clearance because the air traffic supervisor was asleep, safety and aviation officials said Wednesday.

The supervisor _ the only controller scheduled for duty in the tower around midnight Tuesday when incident occurred _ had fallen asleep, said an aviation official, who spoke on condition of anonymity to discuss the incident.

The National Transportation Safety Board is gathering information on the occurrence to decide whether to open a formal investigation, board spokesman Peter Knudson said.

The pilots of the two commercial planes were unable to reach the tower, but they were in communication with a regional air traffic control facility, Knudson said. That facility is in Warrenton, Va., about 40 miles from the airport.

Regional air traffic facilities handle aircraft within roughly a 50 mile radius of an airport, but landings, takeoffs and planes within about three miles of an airport are handled by controllers in the airport tower.

After pilots were unable to raise the airport tower by radio, they asked controllers in Warrenton to call the tower, Knudson said. Repeated calls to the tower went unanswered, he said.

The planes involved were American Airlines flight 1012 and United Airlines flight 628T, Knudson said.

The Federal Aviation Administration released a statement confirming the incident.

"The FAA is looking into staffing issues and whether existing procedures were followed appropriately," agency spokeswoman Laura Brown said in an email.

It's unlikely the safety of the planes was at risk since the pilots would have used a radio frequency for the airport tower to advise nearby aircraft of their intention to land and to make sure that no other planes also intended to land at that time, aviation safety experts said. At that time of night, air traffic would have been light, they said.

Also, controllers at the regional facility, using radar, would have been able to advise the pilots of other nearby planes, experts said.

The primary risk would have been if there were equipment on the runway when the planes landed, they said.

But the incident raises serious questions about controller fatigue, a longstanding safety concern, said John Goglia, a former NTSB board member.

"You have to watch your schedules to make sure (controllers) have adequate rest," Goglia said. "It's worse when nothing is going on. When it's busy, you have to stay engaged. When it's quiet, all they have to be is a little bit tired and they'll fall asleep."

Aviation experts emphasized the unusual nature of the incident.

"I'm not sure that in all the years I've been flying airplanes that I can recall coming into a major airport and I couldn't get hold of a controller in the airport tower," said aviation safety consultant John Cox, who spent 35 years as an airline and corporate pilot.

However, planes, including smaller airliners, land all the time at small airports that don't have control towers or controllers to clear landings.