The single-engine plane that crashed into the South Lawn of the White House early Monday managed to elude the elaborate security cordon that guards the president and his family.

Since drunken crowds broke up the furniture to celebrate Andrew Jackson's inauguration in 1828, a fairly steady stream of uninvited guests has intruded on the White House and its 13 acres of lawns and gardens.No president or family member has been harmed on the White House grounds, and intruders who manage to jump the 10-foot-high wrought-iron fence have been quickly captured.

But the Secret Service, assigned to protect the president, has become increasingly wary of aerial intruders since Feb. 17, 1974, when a disgruntled soldier hijacked a helicopter from Fort Meade, Md., and landed on the South Lawn just yards away from where the plane crashed on Monday.

Secret Service guards fired shotgun blasts at the helicopter and the pilot. Army Pfc. Robert Preston, 20, was rushed by security men and apprehended. The 1974 incident occurred at 2 a.m. - the same time as Monday's crash.

White House security precautions were stepped up after the October 1983, bomb attack on the Marine barracks in Beirut when 240 Marines were killed and the level of threats against U.S. officials shot up.

Three-foot-high concrete flower-pot-barricades were placed around the perimeter of the White House. In addition, bomb-sniffing dogs check each vehicle entering the grounds, airport-like metal detectors are placed at every entrance, checking both visitors and staff members, and electronic detectors and cameras cover the lawns.

The airspace over the White House is restricted. The Secret Service operates a control center in the Old Executive Office Building next to the White House to monitor air traffic that veers from the rigid flight path laid down for flights to National Airport, located about two miles southwest on the Virginia side of the Potomac River.

Police sources said the plane that landed on the lawn Monday glided in with its motor dead, presumably at a very low altitude, which may have made radar useless in detecting it.

The FAA's policy is to warn aircraft that veer from the flight path to stay away from the restricted area, which roughly covers the grassy area that ranges from the river less than a mile to the White House.

Terrorist threats since the late 1970s led to the establishment of a ground-to-air missile defense system operated by the U.S. Army on the roof of the adjacent Old Executive Office Building, according to Defense Department officials.

While they refuse to say what weapons they deploy, both the shoulder-held heat-seeking Stinger and Redeye missiles fit the bill.

Army experts said that if an aircraft strays in the direction of the White House, the Secret Service has less than a minute to decide whether to down it.