SAN SALVADOR, JAN. 5 -- President Jose Napoleon Duarte said today that leftist rebels carried out the October murder of the head of the nongovernmental Human Rights Commission, a crime that set off a wave of national and international protests.

Duarte said the case was broken because of the "providential" Dec. 23 arrest of a student, Jorge Alberto Miranda, 19, as he tried to burn a truck in the center of the capital. During interrogation, he admitted being involved in the Oct. 26 killing of Herbert Anaya, the president said.

Miranda said in a brief interview with a local television station last night that he was a guerrilla and had participated in the murder of Anaya. He made the statements after giving four hours of court testimony before being taken to Mariona prison.

Miranda's family and Anaya's commission challenged the account.

"We catagorically reject this show by the government," said a spokeswoman for the commission. "Tomorrow we will present evidence that it was the security forces." The government and U.S. Embassy say the commission is linked to the insurgents.

Anaya, 33, was shot point-blank as he prepared to take his children to school. He had been jailed for 10 months on charges of links to the guerrillas and was freed in February 1986 in an exchange for an Air Force colonel kidnaped by the rebels. His death sparked a week of antigovernment protests by groups saying the government was responsible.

"This confirms our original thesis that the left did it so they could break off peace talks and try to blame the govenment," an adviser to Duarte said. Immediately after the murder, the insurgents broke off negotiations on a cease-fire in the 8-year-old civil war, saying the assassination proved Duarte did not respect human rights.

Duarte, in a press conference, showed arms reportedly turned in by Miranda, maps of the getaway and sketches of three men Miranda said helped carry out the killing.

The investigation was carried out by the Special Investigative Unit, funded by the United States.

"We are convinced these are the people that killed Anaya," Duarte said. "This was a crime by the FMLN" -- the Spanish acronym for the Farabundo Marti National Liberation Front.

"This is important because the murder was presented to the whole world as a death-squad killing," Duarte said, referring to the paramilitary right-wing groups that killed thousands of civilians in the early 1980s. "Before, people would kill anyone for 100 colones {$20} and no one said anything. But now we are proving we can investigate crimes and those who kill cannot act with impunity."

According to Miranda himself and Duarte, Miranda and the others who allegedly participated in the killing were members of the special forces of the Revolutionary People's Army, one of the most hard-line factions of the FMLN.

Duarte said he was "absolutely certain" no torture or coercion had been used to get Miranda to confess. But his father, Jose M. Miranda, said the son complained when visited of not being allowed to sleep for long periods of time. The father added that he appeared hungry.

Sleep and food deprivation are among the most common human rights abuses cited by human rights groups that monitor El Salvador.

"I do not know what they did to him to make him take responsibility for something he did not do, but he could not have done it," said his sister Ana Gladys, 22. "He studied late the night before the attack and was still in bed when it happened. I saw him there and had to get him up so he would not be late for class because he had two final exams that day."

Miranda's mother, Domitila, 49, supported Ana Gladys' version, saying her son was in bed at 6:30 a.m., the time of the killing.

But Justice Minister Julio Alfredo Samayoa said key elements of Miranda's testimony were corroborated by five witnesses, other evidence and lie detector tests.

On television, Miranda said, "There were various reasons for killing him. First, he was not doing his job at the Human Rights Commission well. Second, he was in the hands of the authorities for a long time, he was burned. Third, because he was passing information to the armed forces, and fourth, to be able to blame the armed forces."

Miranda said he provided security, while another man known as Jose drove the truck used and a man called Carlos did the shooting.

Duarte and Samayoa also said two "common criminals" had been arrested for the Dec. 9 killing of Rene J. Cardenas, head of the government's Human Rights Commission in eastern San Miguel province. Samayoa said the police had matched the bullet to a gun found on the two men after they had stolen a vehicle.