President Jose Napoleon Duarte today ordered an Air Force investigation team to determine whether the helicopter crash that killed four of his top Army field commanders and 10 other men was the result of mechanical failure or hostile fire.

Duarte said that the preliminary reports he had received about the crash yesterday in guerrilla-infested eastern Morazan province led him to believe it was a result of a mechanical accident and not guerrilla action as the rebels' Radio Venceremos has claimed in repeated broadcasts.

Speaking at a news conference at his presidential palace that he had called to express his sorrow at the death of Lt. Col Domingo Monterrosa, his top field commander, and three other key officers, Duarte said the loss to him and the nation was "immense" but that it would do nothing to change his policies.

The other three officers killed in the crash were Lt. Col. Napoleon Calitto, Maj. Armando Azmitia and Maj. Nelson Alejandro Rivas.

Duarte announced the investigation as three Salvadoran Army officers in the northeastern province where the crash occurred said the helicopter's pilot had radioed that he was having mechanical difficulties minutes before the aircraft went down.

The Army high command had said last night that the crash near the town of Joateca in Morazan province was "almost certainly" due to mechanical failure. Monterrosa, the overall commander of Salvadoran armed forces in the eastern part of the country, was directing a major offensive in Morazan.

But Radio Venceremos, the guerrillas' clandestine radio that was the first to report the crash, said rebel machine guns hidden in surrounding hills brought the helicopter down.

Today, Radio Venceremos reiterated the claims, giving a detailed description of how rebel gunners set up an ambush, watched three times as Monterrosa's helicopter took off or landed out of the range of their guns around Joateca, then flew into range. Radio Venceremos said their machine-gun fire hit the helicopter's rotors and gasoline tank, causing the craft to explode in mid-air and drop to the ground near the Joateca cemetery.

A Salvadoran Army corporal of the Morazan Battalion, who said he had witnessed the explosion from the town of Osicala, 10 miles southeast of Joateca, said today the helicopter had exploded in mid-air.

In a separate interview in Osicala, 2nd Lt. Pedro Olivares said he had overheard the helicopter's pilot send a radio message saying the aircraft had a "mechanical imperfection" shortly before it crashed.

The pilot "said he couldn't keep it in the air," said Olivares, who had been monitoring radio transmissions. "He said he urgently needed another helicopter for a rescue if his went down."

Lt. Col. Julio Benavides, acting commander in Morazan province, and Lt. Jorge Barahona also said in the provincial capital of San Francisco Gotera that the pilot had sent such a message, although they had not heard it themselves.

Guerrillas attacked a troop truck, fired from rooftops and staged attacks in the central area of San Salvador last night. The small Central American Revolutionary Workers' Party said it was responsible for the attacks.

It seemed possible both that the helicopter was having mechanical difficulties and that rebel fire had brought it down. But the guerrillas also might have learned of the crash by monitoring Army communications, as they are known to do, and then rushed to broadcast a communique falsely claiming that they had downed the helicopter.

Salvadoran and U.S. officials alike had described Monterrosa as the Army's brightest, most promising officer. He was slated to be promoted to full colonel in December, and military sources had said that he was the leading candidate to be named to the currently vacant post of Salvadoran President Orders Probe Of Crash That Killed Key Officers Duarte Says Loss of 4 Commanders Will Not Change His Policies By Loren Jenkins and Robert J. McCartney Washington Post Foreign Service

SAN SALVADOR, Oct. 24 -- President Jose Napoleon Duarte today ordered an Air Force investigation team to determine whether the helicopter crash that killed four of his top Army field commanders and 10 other men was the result of mechanical failure or hostile fire.

Duarte said that the preliminary reports he had received about the crash yesterday in guerrilla-infested eastern Morazan province led him to believe it was a result of a mechanical accident and not guerrilla action as the rebels' Radio Venceremos has claimed in repeated broadcasts.

Speaking at a news conference at his presidential palace that he had called to express his sorrow at the death of Lt. Col Domingo Monterrosa, his top field commander, and three other key officers, Duarte said the loss to him and the nation was "immense" but that it would do nothing to change his policies.

The other three officers killed in the crash were Lt. Col. Napoleon Calitto, Maj. Armando Azmitia and Maj. Nelson Alejandro Rivas.

Duarte announced the investigation as three Salvadoran Army officers in the northeastern province where the crash occurred said the helicopter's pilot had radioed that he was having mechanical difficulties minutes before the aircraft went down.

The Army high command had said last night that the crash near the town of Joateca in Morazan province was "almost certainly" due to mechanical failure. Monterrosa, the overall commander of Salvadoran armed forces in the eastern part of the country, was directing a major offensive in Morazan.

But Radio Venceremos, the guerrillas' clandestine radio that was the first to report the crash, said rebel machine guns hidden in surrounding hills brought the helicopter down.

Today, Radio Venceremos reiterated the claims, giving a detailed description of how rebel gunners set up an ambush, watched three times as Monterrosa's helicopter took off or landed out of the range of their guns around Joateca, then flew into range. Radio Venceremos said their machine-gun fire hit the helicopter's rotors and gasoline tank, causing the craft to explode in mid-air and drop to the ground near the Joateca cemetery.

A Salvadoran Army corporal of the Morazan Battalion, who said he had witnessed the explosion from the town of Osicala, 10 miles southeast of Joateca, said today the helicopter had exploded in mid-air.

In a separate interview in Osicala, 2nd Lt. Pedro Olivares said he had overheard the helicopter's pilot send a radio message saying the aircraft had a "mechanical imperfection" shortly before it crashed.

The pilot "said he couldn't keep it in the air," said Olivares, who had been monitoring radio transmissions. "He said he urgently needed another helicopter for a rescue if his went down."

Lt. Col. Julio Benavides, acting commander in Morazan province, and Lt. Jorge Barahona also said in the provincial capital of San Francisco Gotera that the pilot had sent such a message, although they had not heard it themselves.

Guerrillas attacked a troop truck, fired from rooftops and staged attacks in the central area of San Salvador last night. The small Central American Revolutionary Workers' Party said it was responsible for the attacks.

It seemed possible both that the helicopter was having mechanical difficulties and that rebel fire had brought it down. But the guerrillas also might have learned of the crash by monitoring Army communications, as they are known to do, and then rushed to broadcast a communique falsely claiming that they had downed the helicopter.

Salvadoran and U.S. officials alike had described Monterrosa as the Army's brightest, most promising officer. He was slated to be promoted to full colonel in December, and military sources had said that he was the leading candidate to be named to the currently vacant post of chief of the Army to oversee field operations nationwide.

Several officers and soldiers who had been commanded by Monterrosa today expressed sorrow about his death but termed it a normal cost of the war.

"We're accustomed to people dying," Lt. Barahona said. "Anything can happen in war."

A small Army band played somber tunes outside the church in San Francisco Gotera this morning before a memorial mass for two of the dead field commanders who had their headquarters in the provincial capital. Soldiers holding rifles mingled with peasants in the crowded church.

On another matter, Duarte reprimanded the U.S. Embassy publicly for having issued a communique two nights ago condemning the apparent death-squad killing of the 14-year-old son of a Salvadoran labor official without checking with the Salvadoran government.

Duarte said investigations launched after the U.S. communique was issued had raised doubts about the story. He said the boy's father, Alirio Montes, had retracted the story, although Montes, who told reporters yesterday of finding the bodies of his son and two other youths, had apparently gone underground out of fear for his life.

The U.S. Embassy said today they had nothing to add to the story, but officials said they were checking