The judge in the murder trial of six Salvadoran former guardsmen released one of them today, shifting his status to witness in the 1980 killing of four American churchwomen.

President Jose Napoleon Duarte, meanwhile, said that on the basis of lie-detector tests of the guardsmen, "I am personally very much convinced there was no participation" in the crime by "anybody else" higher up the chain of command. The highest rank among those accused of killing the three nuns and a lay worker was that of the squad leader, a sergeant.

Judge Bernardo Rauda Murcia ordered the five former guardsmen held on murder charges. Salvador Rivero Franco, who was shifted to witness status, allegedly was with the others when they stopped a van carrying the women away from the San Salvador Airport Dec. 2, 1980. But their jeep broke down and Rivero Franco was left with it while the others went on and committed the crime, according to an earlier account by Duarte.

Duarte said the case of the five accused is now in the hands of the court, but he reiterated his "moral certainty" expressed Thursday to a business group "that these men are guilty." They were dismissed from the National Guard in order to be tried before a civilian court.

The judge now begins a formal investigation into the value of the evidence, after which he is expected to recommend a trial. "We are sure they are guilty," said the defense minister, Gen. Jose Guillermo Garcia, in an interview. "There is no need to worry about it."

Duarte announced the judge's action personally to a hastily called conference of the foreign press corps, many of whom had spent most of the day unsuccessfully trying to learn from civil authorities what had happened. The situation underlines the difficulty that Duarte's government is having in convincing bureaucrats and lower-ranking military officers of the importance of showing progress in correcting human-rights abuses in El Salvador.