Washington (CNN) -- Two planes landed safely early Wednesday morning at Washington's Reagan National Airport after they were unable to reach anyone at the airport's air traffic control tower, according to the FAA and the National Transportation Safety Board.

The FAA would not comment on a media report that the airport controller had fallen asleep. NTSB spokesman Peter Knudson said, "All we know is the controller was unresponsive and we want to know why."

The situation began at 12:10 a.m. Wednesday when an American Airlines plane attempted to call the tower to get clearance to land and got no answer, Knudson said. The plane had been in contact with a regional air traffic control facility, and a controller at that facility advised the pilot that he, too had been unable to contact anyone at the tower, according to a recording of air control traffic at the website liveatc.net.

"1012," the controller said, referring to the airline's flight number, "called a couple of times on landline and tried to call on the commercial line and there's no answer.

"The tower is apparently unmanned," the controller said.

Apparently asked why by a pilot, the controller later responded, "Well, I'm going to take a guess and say that the controller got locked out. I've heard of this happening before. Fortunately, it's not very often," he said.

Knudson said the plane landed without incident in a situation termed an "uncontrolled airport."

About 15 minutes later, a United flight also failed to reach the tower and landed without any problems, he said. After that, the controller in the tower was back in communication. Knudson said one controller was staffing the tower at the time this occurred.

FAA spokeswoman Laura Brown said the agency is investigating what happened. "The FAA is looking into staffing issues and whether existing procedures were followed appropriately."

Transportation Secretary Ray LaHood on Wednesday ordered the FAA to schedule two controllers on the overnight shift.

"It is not acceptable to have just one controller in the tower managing air traffic in this critical air space. I have also asked FAA Administrator Randy Babbitt to study staffing levels at other airports around the country," he said.

Knudson said it's not uncommon for planes to land at airports in such a situation. He said control towers at some fields around the country shut down for the night and planes still land. However, he could not comment on whether that practice was ever used at Reagan National.

Knudson did not have information on how many passengers and crew were aboard the two planes involved in the situation Wednesday morning.