An investigation of the helicopter crash that killed the Salvadoran armed forces' top field commander and three other senior officers has determined that it was caused by a bomb, reliable Salvadoran military and foreign sources said yesterday. They said they believed the bomb was placed by leftist guerrillas.

The well-informed sources, who declined to be named, said an investigation of the crash that killed Lt. Col. Domingo Monterrosa and 13 other people Oct. 23 has determined that the UH1H helicopter had exploded in midair from a bomb probably put into the craft moments before it took off.

At the time of the crash the guerrillas said they had shot the helicopter down with heavy machine guns. Both the Salvadoran armed forces and the government said at the time that "mechanical failure" of the U.S.-supplied helicopter was the cause of the accident.

Washington Post correspondent Loren Jenkins quoted sources close to the official investigation as saying that it seemed a bomb had been put on the helicopter in the luggage of one of the passengers.

According to these sources, witnesses who saw the crash reported a midair explosion shortly after the helicopter had lifted off from the town of Joateca in Morazan province.

The death of Monterrosa was a major psychological blow for the Salvadoran armed forces and deprived President Jose Napoleon Duarte of one of his most effective suporters within the armed forces.

As a result of the investigation report, which is not expected to be made public for about a week, the Salvadoran Air Force has drastically revised its security measures around helicopters and planes.