HEYENNE MOUNTAIN, Colo., Sept. 26 — President Bush has authorized two midlevel Air Force generals to order commercial airliners that threaten American cities shot down without checking first with him, a senior military officer said today.

The senior officer, Gen. Ralph E. Eberhart of the Air Force, the head of the North American Aerospace Defense Command, said in an interview that such life-or-death decisions would be made by the generals only as a last resort when an attack was seconds away and there was not enough time to consult with General Eberhart, a four-star officer, or the president.

Vice President Dick Cheney revealed this month that in the hours after the attacks on the World Trade Center and the Pentagon, Mr. Bush had ordered the downing of any passenger jets that imperiled Washington. But days after the Sept. 11 hijackings, Mr. Bush, Defense Secretary Donald H. Rumsfeld and the Joint Chiefs of Staff approved new rules of engagement that reflected the heightened concern over possible new terrorist strikes and how to confront them swiftly, General Eberhart said.

Before the attacks there were no formal rules on how the military should deal with an airliner hijacked over the United States, flown by what in essence are suicide bombers.

"If there's time, we'd go all the way to the president," said General Eberhart, who also leads the United States Space Command. "Otherwise, the standing orders have been pushed down to the regional level."

Maj. Gen. Larry K. Arnold, a two- star officer at Tyndall Air Force Base, Fla., would have that authority for the continental United States. Lt. Gen. Norton A. Schwartz, a three- star officer at Elmendorf Air Force Base, Alaska, would have authority for Alaska. Hawaii is covered by the United States Pacific Command, headed by Adm. Dennis Blair, instead of NORAD.

Citing security concerns, General Eberhart declined to sketch a course of events that would result in the decision to down a civilian airliner being made by someone other than the president.

The change in the rules of engagement regarding shooting down civilian aircraft is part of the rethinking of the North American Aerospace Defense Command, better known as Norad, which was born during the cold war and has always been oriented toward external threats. For more than 40 years in a bunker deep inside this granite peak, elite Norad specialists with early-warning radars have peered out over America's borders to alert the nation to an incoming enemy air strike.

But on Sept. 11, that vaunted defense turned out to be a modern Maginot line, blind to terrorist attacks originating in the United States that war planners never dreamed could pose a threat.

"If somebody had called us and said, we have a hijacking 100 miles out coming from Europe or South America, there are terrorists on board and they've taken over the airplane, that's a scenario we've practiced," said General Eberhart, a Vietnam veteran. "We did not practice — and I wish to God we had — a scenario where this takes off out of Boston, and minutes later crashes into New York City. This is a whole new ballgame."

Since the attacks, commanders at Norad's nerve center here have quickly turned their sights to new threats inside the country.

More than 100 fighter jets at 26 bases nationwide stand ready to take off on 10 minutes' notice, up from 14 planes at seven bases on comparable alert the day of the attacks. F-15's and F-16's fly round-the-clock over Washington and New York, and randomly over dozens of other cities. Last Sunday, fighters flew over several National Football League games although officials would not say which ones.

Since Sept. 11, there has been no such thing here as a routine in-flight problem. Any commercial airliner with a radio failure or a silent transponder is immediately suspect, and fighters have been scrambled several times in the last two weeks to investigate what turned out to be false alarms.

"Everyone is very twitchy right now," said Brig. Gen. J. D. Hunter, a Canadian Air Force officer who is vice commander of the mountain's operations center.

Some Federal Aviation Administration radars are not compatible with Norad military radars that gaze out 200 miles beyond United States territory, General Eberhart said. So Norad is moving nearly a dozen mobile ground radars around the country to expand its coverage of the interior United States. Awacs surveillance planes also patrol the skies.

At the operations center inside this durable fortress outside Colorado Springs, air battle management officers, as they are called, monitor giant multihued radar images for the tell-tale blip of incoming attacks. But the officers also have new computers whose screens display a tiny turquoise dot for each of the thousands of commercial and private flights the F.A.A. is tracking at any given moment. A few keystrokes yields information on any of them.

Norad has also opened a direct telephone line to the F.A.A. If a problem arises, Norad officials here and at regional commands quickly hold a teleconference with aviation officials to assess the situation. The aviation administration now has a liaison in the Norad operations center.

"We've improved our ability to communicate with the F.A.A.," said Brig. Gen. Michael C. Gould of the Air Force, the operations center commander. "We really never had the need to respond like this before."

Indeed, the air defense mission here is at the forefront of the renewed focus on homeland defense. At the height of the cold war, air defenses under joint Canadian and American control operated 3,600 fighter jets. But with a declining threat, shrinking Pentagon budgets and higher-priority missions to monitor ballistic missile launchings around the world, Norad's air defense role dwindled to the 20 fighters — 14 in the continental United States — on alert two weeks ago.

Norad's fight against terrorism is coordinated from inside a cavernous complex bored 1,700 feet into this mountainside. When the bunker was completed in 1966, it was designed to withstand a 31-megaton Soviet nuclear strike.

American officials concede that the far more destructive weapons now available would turn this mountain into a valley if it suffered a direct hit.

But commanders persist in keeping up appearances. A pair of 25-ton steel doors swung open to allow the first public visitor since the terrorist attacks to enter a 4.5-acre city of 12 three-story buildings erected on a metal base sitting atop giant metal coils. Designers figured that if the Big One ever hit, the command post would sway on its massive shock absorbers and lead a retaliatory strike.

Nowhere is the contrast between the pre-attack and post-attack visions more striking than in the command's air warning center, which before Sept. 11 was a quiet, three- person office tucked away down one of the many corridors in the labyrinthine subterranean complex.

The office has been rechristened the Norad Battle Management Center, and bustles with more than 40 military specialists who track weather, monitor security at the air bases where fighters are stationed, and evaluate the effects of potential attacks from chemical or biological weapons.

Other crews monitor not only the 7,000 daily flights into the United States from abroad, but also any domestic flights of concern.

"It's good if it keeps us more on top of what's happening," said Staff Sgt. Claudette Johnson, 31, an instructor pressed into service to monitor the air-traffic control chatter on the open line to the F.A.A. "Advanced notice can never be a bad thing."

General Eberhart and other commanders say there is no let up in sight. They expect air combat patrols to secure the skies over major sporting attractions, like the World Series, as well as other big events, like launchings of the space shuttle.

"We're trying to game this out," said General Gould, "and anticipate where terrorists could strike next."