U.S. military authorities received notification that commercial airliners had been hijacked minutes before the planes crashed into the World Trade Center towers and the Pentagon on Tuesday, but in each case, the notification came too late for interceptor aircraft to get into position over New York and Washington in time to make a difference, defense officials said yesterday.

Questions about the time it took U.S. military planes to respond to the threat of several hijacked aircraft speeding toward the nation's financial and military centers have dogged the Pentagon since the attacks. Gen. Richard B. Myers, vice chairman of the Joint Chiefs of Staff, was pressed by a Senate panel about the reaction times of military interceptors during a hearing Thursday on his nomination to become chairman. And the matter has stood as emblematic of the U.S. government's overall lack of preparedness for the terrorist assault.

Top Pentagon officials have been slow to respond to press inquiries for a time line that would establish the exact times that civil aviation authorities became aware of the hijackings, when U.S. military commanders were notified and when U.S. fighter jets took to the air. At least part of the Pentagon's reluctance to disclose the information has been explained by a spokesman for Defense Secretary Donald H. Rumsfeld as resistance to discussing any operational information.

But three senior officers with access to information about the response times filled in details yesterday, saying they showed that military planes never stood a chance of blocking the attacks.

At the time of the terrorist strikes, the North American Aerospace Defense Command (NORAD), which has charge of defending the United States against air attack, kept fighter jets on alert at only seven bases around the country. The planes and their pilots were primed to be in the air within 15 minutes of an order to scramble.

NORAD received word at 8:38 a.m. that a United Airlines flight had been hijacked, and six minutes later, two F-15 fighter jets were ordered into the air from Otis Air Force Base on Cape Cod. But two minutes later, the United plane crashed into the first New York tower.

The military jets were airborne at 8:52. When a second United plane hit the other tower at 9:02, they were still 70 miles away from Manhattan, officials said.

Military authorities learned that still another hijacked plane might be headed toward Washington several minutes after the second attack in New York. Two F-16 fighter jets took off from Langley Air Force Base in Virginia at 9:35, but the American Airlines plane that roared into the Pentagon struck two minutes later.

Even if any of the fighter jets had gotten over New York or Washington ahead of the hijacked planes, several senior Air Force officers voiced doubts that the military pilots would have had time to run through standing procedures before shooting down the commercial airliners.

While military air defense crews had practiced intercepts of hijacked planes, the exercises had tended to assume the aircraft would be outside U.S. borders, over the Atlantic or Pacific, with time to consult the White House before any drastic action was taken.

The F-16s were in position over Washington in time to have intercepted the fourth plane hijacked, the one that crashed in a Pennsylvania forest. Asked if rules of engagement would have allowed the Air Force to shoot the plane down, Deputy Defense Secretary Paul Wolfowitz said yesterday: "I think it was pretty clear at that point that that airliner was not under the pilot's control and that it was heading to do major damage." He said any military intervention would have ultimately been the decision of President Bush.

"We were already tracking that plane that crashed in Pennsylvania," Wolfowitz said in an interview with public television's "Newshour with Jim Lehrer."

"I think it was the heroism of the passengers on board that brought it down but the Air Force was in a position to do so if we had to," Wolfowitz added.

Accounts from telephone calls to relatives from passengers aboard the plane, which had taken off from Newark, N.J., and was bound for San Francisco, indicated they tried to take back control from the hijackers before the plane crashed.

Authorities investigate a Cessna 185 forced down by two F-16 fighters near Martinsburg, W.Va., on Thursday.