WASHINGTON -- As a wayward Cessna flew deep in restricted airspace, national security officials were on the phone discussing whether to implement the last line of defense: shooting it down.

The single-engine Cessna that prompted a frenzied evacuation of the White House, Capitol and Supreme Court on Wednesday veered away from downtown landmarks just before that decision needed to be made.

But it was a close call.

One senior Bush administration counterterrorism official said it was "a real finger-biting period because they came very close to ordering a shot against a general aircraft."

"How many more seconds away or minutes -- it was within a very small window where there would have been the decision," said the official, who spoke only on condition of anonymity because of the sensitivity of the matter.

Lt. Col. Tim Lehmann, one of two F-16 fighter pilots who tracked the Cessna, said he was prepared to use deadly force. He said he realized how serious the situation became when he looked at the Cessna and saw the Washington Monument in the background.

"We may have been on the cusp of some kind of engagement," Lehmann said.

"I don't know how close we came."

A response system put in place after the attacks, coordinated in part by the Homeland Security Department's classified operations center, alerted other areas of the federal government to the incoming plane. Security forces at individual facilities and agencies decided on a case-by-case basis whether to evacuate or raise their alert level.

Alert levels at the White House and the Capitol were raised to their highest level -- red -- at the height of the frenzy.

President Bush, biking at the Patuxent Wildlife Research Center in Beltsville, Md., was unaware of the midday scare as it was occurring. His security detail knew of the raised threat level but did not tell him.

White House press secretary Scott McClellan said that a review of how the situation was handled was being conducted. But he said Bush was not upset that he was not filled in.

"The president has a great amount of trust in his security detail," McClellan said. "If there are any improvements that need to be made, they will be made."

Immediately after the Cessna entered the restricted 30-mile radius Air Defense Identification Zone at 11:28 a.m. EDT, authorities activated the Domestic Events Network to share information as they tracked the plane. The network, a conference call of officials from the Homeland Security Department, Customs and Border Protection, the Pentagon, the Federal Aviation Administration and a handful of other agencies, lasted until the Cessna landed just over an hour later.

Defense Secretary Donald H. Rumsfeld was apprised of the situation as it unfolded. He is among a small handful of top Pentagon officials who can order a shootdown. The president also may give such an order.

Pentagon officials sought to play down the incident, saying the small plane was not seen as a serious threat and did not come close to being shot down. Homeland Security spokesman Brian Roehrkasse declined to comment on how close it was.

Brian Jenkins, counterterrorism analyst for the RAND Corp. think tank, agreed the threat from the Cessna was limited.

"The quantity of explosives that you can pack in a little Cessna is not the quantity of explosives you see placed in these big truck bombs," Jenkins said. "In terms of explosives, it probably could not do that much damage."

However, government officials also had to consider the possibility it was carrying chemical or biological weapons. A relatively small amount of either could have devastating effects.

Customs officials scrambled a Black Hawk helicopter and a Cessna Citation jet at 11:47 a.m. to intercept the plane and were joined a few minutes later by two Air National Guard F-16 fighter jets.

The Cessna pilot appeared confused by the aircraft escort and did not respond to repeated signals ordering the plane to turn away. The F-16s fired four warning flares before the Cessna finally veered west and away from the secure zone. They landed safely at an airport in Frederick, Md.

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