Federal aviation officials are reviewing air traffic controller staffing at airports around the country after two airliners landed at Reagan National Airport without clearance from the airport tower because they were unable to raise anyone there.

An aviation official, who spoke on condition of anonymity to discuss the incident, said an air traffic supervisor - the lone controller on duty around midnight on Tuesday when the incident occurred - had fallen asleep.

The pilots of the two planes were in contact with controllers at a regional Federal Aviation Administration (FAA) facility about 40 miles away in Warrenton, Va., Peter Knudson, a spokesman for the National Transportation Safety Board (NTSB), said Wednesday.

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

Transportation Secretary Ray LaHood, responding to the incident, said in a statement that he has directed FAA to put two air traffic controllers on the midnight shift at Reagan National.

"It is not acceptable to have just one controller in the tower managing air traffic in this critical air space," LaHood said. Reagan National is located in Northern Virginia just across the Potomac River from Washington.