WASHINGTON (AP) _ When Nezar Hindawi wanted to blow up an Israeli plane in 1986, he packed his girlfriend's suitcase with three pounds of an odorless explosive called Semtex and sent her to London's Heathrow airport to board an El Al Boeing 747.

The bomb, which was pre-set to detonate, passed through an airport X-ray machine, and it was discovered only after an El Al security guard questioned the young woman, Anne-Marie Murphy, who was unaware she was carrying the explosive device.

Terrorism experts say Semtex, which is one of the substances suspected in the bombing last week of Pan Am Flight 103 over Scotland, is a terrorist's dream. So far, it has defied all detection attempts by X-ray machines, dogs trained to sniff bombs and other devices.

''It's a favorite with these groups because it's significantly harder to detect than anything else - it's more powerful than other plastics,'' said George Carver, a senior CIA analyst in the 1960s and 1970s who now is a researcher at the Center for Strategic and International Studies, a Washington think tank.

Like its American cousin, C-4, Semtex is malleable like putty and shows up on an X-ray operator's screen as an indistinct, dense mass. But unlike C-4 and other known plastic bombs, Semtex is odorless.

Semtex also has the advantage of being stable, meaning it cannot go off unless it is meant to. Terrorism experts, including Neil Livingstone of Georgetown University, say Semtex is so reliable it has been used by Arab terrorists sending letter bombs to Israeli officials.

Technology is being developed to overcome the Semtex threat, and two experimental devices are in use at the Los Angeles and San Francisco airports. The devices shoot neutrons which can interact chemically with plastic explosives in luggage and indicate the existence of a bomb.

Czechoslovakia, which has a well-developed, state-run arms and munitions industry, makes Semtex at a plant called the Eastern Bohemian Works, Livingstone said. State Department officials say they have asked Czech authorities in the past to stop making Semtex or at least to prevent its export.

Recently, Czechoslovakia indicated it might be willing to stop selling Semtex, said Livingstone, who has served as a consultant to government security and intelligence agencies and has written seven books on international security and terrorism.

Czechoslovakia is believed to have supplied Semtex to Syria and Libya, said one State Department official who spoke on condition of anonymity.

In testimony at Hindawi's 1986 trial, British authorities said Syria had supplied the bomb for the El Al plane. The British government subsequently broke off diplomatic relations with Syria over the affair, although ties later were restored.

The State Department official said Libya has supplied Semtex in several shiploads of weapons to the Irish Republican Army in 1986 and 1987 and may have given it to other terrorist groups.

Semtex also is believed to have been used by a tiny Syrian- and Libyan- supported Palestinian group once known as the May 15 organization and which is now known as the Hawari group.

The group is believed to have been responsible for the April 1986 explosion aboard a TWA Boeing 727 flying from Rome to Athens. Four Americans were killed in the blast, which Greek investigators said probably was caused by a Semtex bomb the size of two or three cigarette packs. The bomb was placed under a seat in row 10.

The Hawari group specializes in plastic bombs, according to U.S. counterterrorism experts. One of its operatives, Mohammed Rashid, is being sought by the United States for questioning in the 1982 bombing of a Pan Am jetliner over Hawaii which resulted in one death.