In the cockpit of his F-16 more than two miles over Washington, Lt. Col. Steve Chase knew another fighter was coming at him -- he could see it on radar -- but scanning the blue-gray sky, he couldn't pick up the approaching jet.

The two jets were miles apart but hurtling toward each other at a combined speed of more than 1,200 mph.

"He's on our nose," Chase called to his passenger.

Out of nowhere, an F-15 Eagle appeared and just as quickly soared past, the air-to-air missiles beneath its wings visible as it banked into a turn high above the Washington Monument. The jet joined with other aircraft flying random orbits over the District. They were on combat patrol, ready to shoot down any aerial threat to the nation's capital, including, if necessary, a hijacked airliner filled with civilians.

It was a sight that Chase, a lanky veteran pilot assigned to the D.C. Air National Guard, has witnessed often during the past half-year while flying dozens of combat patrol missions over Washington. But it still leaves him with an odd feeling.

"It is fairly ominous to see an aircraft with live missiles, air-to-air ordnance, and having as a backdrop the Mall, or the White House, or the Capitol," he said.

That's the way it has been since Sept. 11, when Chase and other pilots from the D.C. Air Guard scrambled to get F-16 Fighting Falcons in the air minutes after the Pentagon was hit by a hijacked jet.

From that moment, the military has flown continuous combat patrols over the District and New York, 24 hours a day, seven days a week, as well as random patrols over other cities.

Concerned about the cost and the strain on fighter units, defense officials plan to end the continuous coverage soon and instead fly patrols intermittently. The system will increasingly rely on fighters kept on "strip alert," ready to scramble within 15 minutes.

At Andrews Air Force Base, where the D.C. Air Guard is based, F-16s sit on alert inside four shelters hastily erected after the terrorist attacks. The sleek gray jets, fully fueled and armed with air-to-air missiles, await their pilots with open canopies.

"You hope to God you never have to utilize your services," said Chase, a soft-spoken Calvert County resident.

For some terrifying hours on Sept. 11, pilots from the D.C. Guard thought there was a good chance they would.

Most of the National Guard pilots are part-time warriors who fly commercial jets in their civilian lives.

The D.C. Guard's motto is Capital Guardians, but for years, most of the 121st Fighter Squadron's focus was overseas. In recent years, it has been deployed to enforce the no-fly zone over Iraq, train in Argentina and fly counter-narcotics missions in the Caribbean.

The squadron had just returned three days earlier from a training exercise in Nevada, preparing to deploy a few months later to Turkey. Many of the pilots were in the squadron headquarters at Andrews when the first plane struck the World Trade Center.

They were immediately suspicious. "How do you make a mistake like that?" Lt. Heather Penney recalls thinking.

Then the second plane hit a tower. "We're under a terrorist attack!" someone yelled.

A routine meeting of pilots quickly broke up. "People just launched into action," said Chase, who was at the operations desk. "There was a buzz in the unit. People got on the radio and telephones to higher headquarters."

Unlike other Guard units, the D.C. Guard reports to the president, not a governor. And the 113th Wing works closely with Secret Service agents across the runway in the Air Force One hangar.

At the wing headquarters, Brig. Gen. David F. Wherley Jr.'s first inkling that the attacks would go beyond New York was when one of his officers, whose husband worked at the Pentagon, saw on television that the building had been hit and began shrieking.

"You've got to be strong," Wherley told the officer. The general then raced out of the building, running several hundred yards to squadron headquarters.

"The Secret Service calls up and says, 'Hey, get in the air,' " Lt. Col. Marc Sasseville recalled.

"We have to get some instructions," Wherley told squadron officers. "We can't just fly off half-cocked."

The general went to work through classified communications channels and within a half-hour received oral instructions from the White House giving the pilots extraordinary discretion to shoot down any threatening aircraft.

"They said challenge them, try to turn them away; if they don't turn away, use whatever force is necessary to keep them from hitting buildings downtown," Wherley said.

Three of the squadron's jets had been flying that morning, returning to Andrews after a training flight to North Carolina.

Only one of the planes had enough fuel to keep flying, and it had no missiles, just training ammunition. Maj. Billy Hutchison, who landed before a decision had been made, was quickly ordered back into the air, launching about 50 minutes after the Pentagon was hit.

Monitoring radios in the operations room, officers could hear the Federal Aviation Administration closing airspace across the country and ordering all planes to land, concluding with a warning that violators would be shot down.

The words chilled Wherley. "I guess that will be us doing the shooting," he remembers thinking.

The wing learned that there were about a half-dozen suspicious aircraft in the air across the country, among them hijacked United Airlines Flight 93, on a path toward Washington.

"Nobody knew it had crashed. We just knew there was an airplane out there that could be coming to Washington," Wherley said. "We knew the threat was real."

By then, Hutchison was roaring up and down the Potomac River, flying a combat patrol over the Pentagon. Jets had scrambled from Langley Air Force Base in Virginia and were also over the region, at a much higher altitude.

On the radio, squadron officers warned Hutchison of the Flight 93 threat: Look northwest of town, toward Georgetown, he was told.

Penney and Sasseville were also airborne looking for an inbound jetliner. "We didn't know what we were looking for -- how high he was coming, or low, or where he was going," said Sasseville, a Vienna resident.

Lacking missiles on their jets, the pilots wondered how they would stop the airliner. "Where do you shoot at an airliner to bring it down?" Sasseville recalled thinking.

It was a decision they did not have to make. Flight 93 crashed in Pennsylvania, apparently after passengers tried to take control of the plane from the hijackers.

In the days that followed, as the military hastily cobbled together a system of patrols, the situation remained chaotic. After civilian air traffic resumed, the pilots struggled to cope with the possibility that they might have to shoot down a passenger jet.

"For the pilots, it's a lose-lose situation," Sasseville said. "If there was a hijacked airliner heading to D.C. and you don't shoot it down and it hits the monuments, you look like a goat. If you do shoot it down, you've killed a bunch of passengers."

In the early weeks, the pilots would often spot aircraft on radar that controllers on the ground and in the air had not yet seen.

"Just about anything flying was looked at very closely, and it kept us on our toes," Chase said. "It kept the mission from being anything but boring. They were fairly exciting for the first month or so, until things started slowing down."

The patrols began settling into a routine. Radar improvements, better communication with all aircraft and tighter coordination among the military and federal agencies meant fewer surprises.

The mission took a toll, straining jets and maintenance crews, preventing pilots from training for other missions and wreaking havoc on family life. "Somebody's got to fly at 2 in the morning, and that's not a fun time to be in the air, especially in bad weather," Sasseville said.

To the pilots' relief, the D.C. Guard stopped flying regular combat patrol missions several weeks ago, with units from Langley and other bases picking up the slack.

But the squadron's alert mission still requires round-the-clock staffing. On-call pilots stay in a trailer near the tarmac dubbed the Flamingo Inn, complete with pink plastic mascots in the grass.

Several times in recent weeks, the klaxon has sounded, and crews have scrambled to get jets in the air -- not in response to threats but to replace aircraft unable to fly the combat patrols.

For the most part, Chase acknowledged during his recent flight, the combat patrols have become tedious.

Far below, a passenger jet lazily floated in for a landing at Reagan National Airport. A KC-135 Stratotanker passed overhead, ready to refuel the F-15s. Northern Virginia looked like an endless white sea of houses.

"This is the extent of it, right here," said Chase, banking the F-16 through another turn. "You're always at the ready, watching things. Lots of sightseeing, trying to figure out the roads."

Back on the ground, Chase said the flights serve as deterrent.

"I don't think we can allow our edge to drop. I think that's what they would want," he said. "We'd be letting ourselves down, our families down, the country down, if we even for an instant allowed ourselves to get the least bit complacent."

Staff Sgt. John Grabenstein helps Lt. Col. Steve Chase strap in before he takes off from Andrews Air Force Base in an F-16 Fighting Falcon, which is used in patrols.Lt. Col. Steve Chase, right, and Lt. Col. Skip Denman talk near an F-16 that proclaims, "Spirit of 9-11, Let's Roll!"