A congressional source had earlier described the plane as a DC-9 passenger jet.

The plane was first noticed at 1:30 p.m., triggering an “Aircon Red” — or air condition red — alert. E-mail evacuation notices were sent to House and Senate members and staff just after 1:40 p.m., and the all clear, a return to “Aircon Green,” was given about 20 minutes later at 2:02 p.m.

An “Aircon Red” alert is triggered when an airplane enters a restricted zone about 15 miles around Washington — or about three to five minutes of flying time, depending on air speed and other factors — according to the Federal Aviation Administration and a former House aide who dealt with security issues.

Despite the clearance notice from police, alarms were audible one mile away from the Capitol and aides reported that they still did not have access to their offices over an hour after the episode was said to have ended.

“The fire marshal is on [the] scene working on silencing alarms, then reoccupation can begin,” Capitol Police spokeswoman Kimberly Schneider told POLITICO by e-mail.

FAA spokeswoman Diane Spitaliere said the plane was on its final approach to Reagan and approximately 18 minutes away from the airport when the crew mistakenly switched communication to the wrong frequency. The crew was out of contact with air traffic controllers for about 15 minutes as the plane continued its approach, a time when pilots are normally in verbal contact with the tower, she said.

A spokeswoman for the North American Aerospace Defense Command, which coordinates air defense in the United States and Canada, said the plane was only three minutes away from the airport when radio contact was reestablished, at which point the airliner was also a few minutes from the Pentagon and other possible targets.  F-16 fighters had scrambled from Andrews to intercept the plane, but they had not made visual contact, raising the question of whether they were in range to shoot down the plane if needed.

NORAD spokeswoman Stacey Knott said the airliner was flying at approximately 10,000 feet and the F-16 fighters returned to base without contacting the plane.

The incident triggered the evacuation order and a series of e-mails to congressional staff.

“As a precautionary measure, the buildings were evacuated and the Capitol complex was secured as we monitored the flight path,” Capitol Police wrote. “The aircraft was intercepted and began responding to directions from the FAA. The commercial aircraft safely landed at [Reagan] and the Capitol complex reopened. The U.S. Capitol Police is investigating the situation in conjunction with” the Transportation Security Administration.

Despite the “precautionary” nature of the evacuation, the alert, which would normally include loud warnings over public address speakers positioned throughout the Capitol and office buildings for such crises, must have alarmed the few people who worked during the weekend between the 111th and 112th Congresses.

“The Capitol Police are tracking an unidentified incoming aircraft. All persons in the U.S. Capitol, Hart, Dirksen and Russell buildings should evacuate now,” read an alert sent to senators and staff.

A similar note sent to members of the House and staff advised that “an unauthorized aircraft has entered restricted airspace.”

Richard E. Cohen contributed to this report.

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