BY NANCY SAN MARTIN AND CARLOS DADA

SAN SALVADOR - Guatemalan President Oscar Berger on Monday blamed organized crime and corruption for the stunning killings of four jailed police officers who were key suspects in the slayings of three Salvadoran legislators.

A group of heavily armed men wearing prison guard uniforms entered El Boquerón maximum security prison east of Guatemala City late Sunday and shot to death the four police officers in their separate cells, officials said.

Agents from the FBI, which has been asked to assist in the investigation of the Salvadorans' killings, were supposed to have questioned the four police officers on Monday, Salvadoran authorities said.

The police officers' killings added a new twist and fresh blood to the already mysterious slayings of the three legislators, including Eduardo D'Aubuisson, son of the man who founded El Salvador's ruling ARENA Party and was long accused of running death squads during a 1980s civil war that left 75,000 dead.

Officials in Guatemala and El Salvador have said they don't know for sure the motives in either of the two homicide cases but have been hinting that the reasons were more likely purely criminal than political.

''There is a war [among organized-crime groups] that has caused the violent death of four important witnesses who might have contributed much to the process,'' Berger said in Guatemala City.

Authorities were still investigating the slayings of the police officers and their links to the three Salvadoran lawmakers, whose charred, bullet-riddled bodies were found on a rural road about 20 miles southeast of Guatemala City on Feb. 19.

The Salvadorans were members of the Central American Parliament, which represents five nations in the region and meets in Guatemala City. The three victims -- D'Aubuisson, Ramón González and William Pichinte -- also were members of ARENA.

The late Roberto D'Aubuisson, widely alleged to have been the creator of El Salvador's death squads, was accused but never convicted in the 1980 assassination of Salvadoran Archbishop Oscar Arnulfo Romero.

Salvadoran and Guatemalan authorities have pointed to drug trafficking as a possible motive in the consecutive slayings, though there has been no clear evidence linking the lawmakers or police officers to the drug trade.

The killings ''demonstrated that organized crime has infiltrated the highest levels of the National Civil Police in Guatemala,'' Salvadoran Public Security Minister René Figueroa was quoted as saying by The Associated Press.

The four police officers were found shot to death in separate cells at a maximum security prison in Cuilapa, 40 miles east of Guatemala City. They included the head of the National Police organized crime unit and three top investigators.

`SO CORRUPT'

Rodrigo Avila, head of El Salvador's National Civil Police, said in a television interview in San Salvador that while he trusted his Guatemalan counterpart, Erwin Sperisen, ``unfortunately, the Guatemalan police is so corrupt I don't know if he can do something.''

The police apparently were killed to ''silence'' them and hamper the investigation into the Salvadorans' homicides, Avila said.

''I wouldn't like to be in Mr. Sperisen's shoes,'' Avila said. ``Now he has two cases, and this one, the killings of the policemen, is even more complex, because he has to find out the level of involvement of his own police force.''

In addition to being shot, the four police officers appeared to have had their throats slit, Guatemalan deputy prosecutor Candido Bremen said.

PRISON SWITCH

Guatemalan justice officials told The Miami Herald the officers were supposed to have been held in a different prison in Guatemala City for at least 90 days.

''We were shocked to find out that they had been transferred,'' Guillermo Melgar, a spokesman for Guatemala's Supreme Court, said in a telephone interview. ``Why they were moved over the weekend is unclear to us.''

According to Guatemalan newspaper Prensa Libre, the last person to see the police officers alive was their attorney, Alfredo Vasquez, who left the prison at noon Sunday.

''They expressed to me their concern and fear, because many of the [other] inmates were people that they themselves had sent to prison,'' Vasquez said.

ORDERED TO LEAVE

Relatives of other inmates who were in the prison for their weekly family reunions told Prensa Libre that they were ordered out of the building by prison guards about 2:30 p.m.

The relatives said they then saw a vehicle with dark windows enter the prison, as well as three men carrying backpacks. Shots were heard later.

President Berger said the investigation will include a thorough interrogation of the prison guards who allowed the killers ''to go through eight gates in order to reach their victims,'' The Associated Press reported.

SUBSEQUENT RIOT

The police killings sparked a 12-hour riot at the prison as inmates, most of them gang members, took five prison officials hostage in an apparent attempt to avert possible retaliations against themselves for the police officers' killings.

The gang members released the hostages after receiving promises that there would be ''no reprisals,'' Nery Morales, a penal system spokesman told The Miami Herald by phone.

At least two other Guatemalan police officers are wanted for questioning in the Salvadoran legislators' homicides. They remain at large.

Miami Herald correspondent Nancy San Martin wrote this report from Miami with contributions from special correspondent Carlos Dada in San Salvador and Miami Herald translator Renato Pérez.