U.S. Capitol Police Chief Terrance W. Gainer received the page just before 4:30 p.m. Wednesday, one hour ahead of the procession that would bring the coffin of former president Ronald Reagan down Constitution Avenue. A plane headed toward Washington was entering restricted airspace, and federal authorities weren't able to communicate with its pilot.

"The situation was escalating. The aircraft was going 240 knots [about 276 mph] . . . and continued to close in on the Capitol Hill area," Gainer said. No one could say if it was friend or foe.

With the plane 11 miles and 31/2 minutes from the Capitol, Gainer made a decision after conferring with local and federal authorities. "Evacuate the buildings," the chief ordered. Then he dashed out of his cruiser, jumped over a barricade and ran toward the Capitol.

Yesterday, local and federal authorities reviewed that decision and the events that followed in the first evacuation of Capitol Hill since the Sept. 11, 2002, terrorist attacks. Officials praised Gainer's quick response and the performance of his force but said the false alarm revealed communication problems and evacuation difficulties that need to be addressed.

The plane turned out to be a Kentucky State Police aircraft ferrying Gov. Ernie Fletcher (R), a former member of the U.S. House of Representatives, to Washington for Reagan's funeral. The plane's transponder, which transmits identifying information to ground controllers, was broken, according to Brian Roehrkasse, a spokesman for the Department of Homeland Security. The plane was one of several given special permission to land at Reagan National Airport for the funeral.

"We believe the system worked and the appropriate security actions were taken," Roehrkasse said, given that federal security officials were unable to identify the plane.

Gainer's order cleared the Capitol, House and Senate office buildings and the Supreme Court within minutes.

House Majority Leader Tom DeLay (R-Tex.) was inside his office when one of his security officers burst in. "He said we had to get out of the building right now," said Tim Berry, DeLay's chief of staff.

DeLay and other staffers ran. "The cops were grabbing anyone who stopped. They were clear that everyone needed to run," Berry said.

Members of Congress and thousands of staff members poured out the doors as police officers shouted: "This is not a drill! Get out as fast as you can!" As they fled, newscasters tripped over wires, one man ran into a barricade and people anxiously watched the skies. Black Hawk helicopters and Air Force fighter jets were sent to intercept the twin-engine, turboprop plane.

D.C. Police Chief Charles H. Ramsey was driving near the West Front of the Capitol with his assistant chief when he saw the commotion. Then his pager went off: Air Con Red. "I knew then there was an unidentified aircraft in airspace that shouldn't have been there or hadn't properly identified itself," Ramsey said.

Daniel Groves, the Kentucky governor's chief of staff, said at a news conference yesterday that the plane's transponder quit working shortly after takeoff from Greater Cincinnati-Northern Kentucky International Airport.

The pilots had intermittent contact with air traffic controllers, but as the plane approached Washington from the southwest, federal authorities became alarmed because no information was being transmitted by the transponder, Homeland Security officials said. Officials at the Federal Aviation Administration command center in Herndon feared that the transponder might have been turned off by terrorists or a pilot whose plane was being hijacked.

As responsibility for the plane passed to controllers in the Washington area, it took more than five minutes for federal law enforcement authorities here to verify the broken transponder, Homeland Security officials said. It was during that period that Gainer had to make the evacuation decision.

After the evacuation order, Capitol Police officers relayed it over a radio warning system called an "annunciator" in congressional offices and the Capitol. Staff members in some offices have told the Capitol Police that they did not hear the warning or that their annunciators were not working. Gainer said he will address that issue and others related to the evacuation in a report about the incident.