A man detained by the Salvadoran Treasury Police in connection with the May 25 murder of a U.S. military adviser admitted the crime at a press conference in the security force's headquarters today.

"I am responsible," said Pedro D. Alvarado Rivera, 23, an engineering student who a Treasury Police spokesman said was arrested a week ago.

Rivera told reporters he was recruited about a year ago by the radical Popular Liberation Forces guerrilla faction, which publicly claimed the killing of Navy Lt. Cmdr. Albert Schaufelberger III the day after it happened. A Treasury Police communique also accused Alvarado Rivera of killing a right-wing constituent assembly deputy later in the summer.

The U.S. State Department Regional Security Office, which is responsible for aiding the investigation of Schaufelberger's death, has not yet interviewed the suspect, according to sources close to the case.

Treasury Police Chief Col. Nicolas Carranza said U.S. officials helped only "to confirm details" during the preliminary investigation.

"The death of American citizens in El Salvador has been an overriding concern of this mission," said the U.S. Embassy's official statement. "We appreciate the extensive efforts of the government of El Salvador to identify and prosecute the perpetrators of this and other crimes.

"We hope efforts of the Treasury Police in capturing a key suspect in the case will result in justice being done to the murderers of our friend and colleague Al Schaufelberger."

Other crimes in which the embassy has shown particular interest have been the killings of four American nuns in December 1980 and two U.S. agrarian reform advisers in January 1981. Members of the Salvadoran security forces were arrested but neither case has come to trial.

The Treasury Police is the smallest of three security forces that, along with the regular Army, make up the Salvadoran military. Human-rights groups repeatedly have accused the Treasury Police of torture and alleged connections of its members with killings by right-wing death squads.

The commander, Carranza, is known for his right-wing political connections. But under his leadership and that of his predecessor, the organization has gained repute for having the most thorough intelligence network in the country.

Alvarado Rivera was presented to journalists wearing handcuffs and standing in front of a red-and-white guerrilla flag that police said was captured along with him in last Thursday's raid on a San Salvador house. Two other men and one woman were also arrested, the communique said, but it did not link them directly to the Schaufelberger killing or the murder of the deputy.

Alvarado Rivera, whose videotaped confession was also shown at the press conference, told reporters that after he was recruited by the guerrillas last year he was assigned to watch Schaufelberger's movements over the course of a week before the assassination.

"I did not go every day to watch," said Alvarado Rivera. "There were more or less five of us in the group" that he said was charged with carrying out the murder.

Asked why he did not leave the country or go to his faction's mountain strongholds after the killing, Alvarado Rivera said, "My job had been designated here." In his 10-minute talk with the press he said he had never participated in any prior operations, "much less executions."