But the ease with which the pilot, Frank Eugene Corder, had penetrated the no-fly zone that theoretically blankets downtown Washington was highly embarrassing to the Secret Service and unnerving to everyone else. Despondent, unemployed and apparently under the influence of alcohol and crack cocaine, Corder took the plane from a small airport near Churchville, Md., about half an hour’s flying time from Washington. He then flew undetected over the city to the Washington Monument, made an aerial U-turn and aimed the Cessna at the White House. The first alert to what could have been an attack on Bill Clinton’s life came at 1:49 a.m., when a guard spotted the plane zooming over the White House fence. After plowing up a patch of the South Lawn, the plane came to a crumpled stop against the building. Corder died in the crash.

Clinton, his wife, Hillary Rodham Clinton, and their daughter, Chelsea, were never in any danger, but that was only luck: as White House officials explained it, the First Family spent the night across the street at Blair House because the White House heating system was being repaired. (Clinton, awakened by his staff after the incident, reportedly went back to sleep.)

What happens now, obviously, is a top-level investigation into why none of the defenses worked. Although security has been improved since the early 1980s, when terrorists were alleged to be stalking Ronald Reagan, the White House stands revealed as a soft target. The place is protected by barricades, bomb-sniffing dogs and riflemen on the rooftops. Its guardians are also equipped with heat-seeking Stinger antiaircraft missiles to shoot down airborne attackers. But no one would launch a Stinger in anything less than a full-blown national-security alert, because the odds of hitting an unintended target, such as an airliner at nearby National Airport, are good. A microwave Doppler radar system was installed in 1991 to detect parachuting assassins. The Doppler radar worked well during a mock raid by parachuting commandos from the army’s Delta Force and navy SEALs, Newsweek sources said. But the radar did not detect Corder’s plane, the sources said, because it looks straight up.

So Clinton’s only early warning comes from an FAA air-traffic-control radar across the river at National Airport. Newsweek has learned that within the last 60 days the Secret Service successfully tested its plan for protecting the president if the airport warned that a low-flying plane was attacking the White House. But commercial air traffic at National shuts down at 11 p.m., and controllers in the tower were absorbed in other duties when Corder’s Cessna appeared on their scope. The FAA isn’t really in the business of protecting the president from flying suicides, a Secret Service official conceded. That leaves the real question: who is?

How secure is the White House? The Secret Service won’t reveal details, but elements of its security system can be cobbled together. They include the following:

Positioned at each of the eight entrances, the structures have bulletproof glass windows.

It’s aimed directly at the airspace above the White House.

The weapons are reportedly shoulder-fired, heat-seeking Stingers.

Whenever Clinton appears on the lawn or during times of high alert, they’re stationed on the roof.

The canines patrol the grounds regularly and are on call to inspect all suspicious packages and vehicles.

The roving guards carry 9-mm semi-automatic weapons and police batons.

The mechanisms can detect anyone who tries to climb over the barriers.

The detectors are buried just beneath the grass throughout the grounds.