Congressional law enforcement authorities activated emergency evacuation plans for the Capitol complex for the second time in less than a year when an apparently stolen aircraft entered restricted airspace in the District of Columbia on Wednesday afternoon.

The incident began at 11:50 a.m., when a radar network monitored by Capitol Police showed an unidentified aircraft had entered restricted airspace, at a distance of about 17 miles from the Congressional campus.

Along with the Capitol Police, both the Federal Aviation Administration and the Secret Service monitored the aircraft — identified as a small, two-seat Cessna 152 — Chief Terrance Gainer said, but the plane’s pilots did not make any attempt to communicate with the authorities.

“The pilot did not maintain or make communication with anyone,” Gainer said.

Law enforcement officials elected to activate the Capitol’s air-alert system at 11:58 a.m., Gainer continued, as the aircraft continued toward the Capitol on a southbound trajectory.

When the plane closed in to within about 4.5 miles of the Capitol, police officials raised the alert to its highest levels, prompting the evacuation of the Capitol and House and Senate office buildings. At approximately the same time, the White House evacuated its facility.

The Capitol Police began with the evacuation of House and Senate leadership, as well as other “continuity of government” officials, Gainer said, and then initiated a full-scale evacuation of all lawmakers, staffs and visitors.

“I think we acted prudently with the information we had,” Gainer asserted of the evacuation.

The chief stated that the evacuation of 10,000 to 15,000 people, and possibly more, was complete within about five to six minutes.

During the same period, Gainer said, military aircraft forced the Cessna to land at an airfield in Frederick, Md., where federal officials took two suspects into custody.

The plane, with tag number N5826G, originated from the Vintage Aeroclub in Smoketown, Pa., but neither of the men, both white males, appear to have had permission to use the aircraft.

“They were not authorized to take the plane from the airport, but in fact did,” Gainer said. Both the FAA and Secret Service officials are investigating the incident, he added.

It is not immediately clear whether the incident was an attempted terrorist attack, and authorities could not comment on the two pilots’ motivation.

“We don’t know what it was,” Gainer said.

The plane’s size, altitude and direction were all taken into account when evacuating the Capitol complex, Gainer acknowledged. “It was inbound from the north, in a line that caused concern,” he added.

Although many details are still preliminary, Gainer said the plane was “going slow” at the time it was intercepted.

FAA authorities said two F-16 fighter jets and a Blackhawk helicopter were dispatched to intercept the aircraft, shortly after it violated the air defense identification zone.

“Flares were displayed in an attempt to get the attention of the pilots,” Gainer said, noting the action seemed to have no immediate impact on the plane’s direction.

Lawmakers began returning to the Capitol at approximately 12:30 p.m., and police allowed visitors and staff to return to their offices at approximately 12:40 p.m., several minutes after authorities confirmed the suspect aircraft had landed at Frederick, Md.