SAN SALVADOR, DEC. 28 -- The Salvadoran government said today that the 1980 assassination of archbishop Oscar Romero, which helped spark the nation's civil war and caused worldwide outrage, would remain unresolved because of a Supreme Court ruling that the testimony of a key witness was not valid.

The ruling has caused an uproar in El Salvador, escalating the already heated presidential campaign rhetoric between the Christian Democrats of President Jose Napoleon Duarte and the rightist Republican Nationalist Alliance (ARENA), the opposition party that controls the National Assembly.

"In fact, the case is now closed and justice has been mocked," said Roberto Giron Flores, who as attorney general was investigating the case until being fired Friday by the ARENA-controlled assembly.

According to diplomatic and political sources, U.S. officials -- who invested much time and effort in the case -- were outraged by the ruling and were making that displeasure known, while acknowledging that nothing could be done to revive the case.

Romero, a charismatic, outspoken opponent of rightist death squads and military repression, was killed by a single shot through the heart on March 24, 1980, while saying mass. It was the most publicized killing of thousands carried out by the death squads. Duarte had vowed to punish those responsible.

Former U.S. ambassador Robert E. White charged the killing was ordered by Roberto d'Aubuisson, founder of ARENA and currently a deputy in the National Assembly. D'Aubuisson has denied the charge.

In November 1987, Duarte made public testimony of the driver of the getaway car in the Romero killing, Amado Garay, who said he worked for Alvaro Saravia, a cashiered Air Force captain and protege of d'Aubuisson. Garay said that after the killing, Saravia reported on the success of the operation to d'Aubuisson.

D'Aubuisson said at the time that Garay was lying, and rejected the allegations.

Based on Garay's testimony, after he passed numerous lie-detector tests and other elements of his testimony were investigated, the Salvadoran government, with U.S. help, asked for the extradition of Saravia from Miami, where he was living illegally.

Saravia was arrested in Miami for violating his visa and has been held while the extradition request proceeded. He was to have been extradited in early January, according to legal sources.

But on Dec. 21 the Salvadoran Supreme Court, described by knowledgeable diplomats and Christian Democrats as dominated by the far right, ruled there were no grounds for arresting Saravia if he returned to El Salvador, and that the extradition request itself was illegal.

The court ruled that delay in presentation of Garay's evidence, after "seven years, seven months and 24 days, completely undermines the credibility of his testimony" and resulted in no evidence linking Saravia to the killing.

"The problem here is that we have to understand the judicial system has been totally dominated by ARENA, which blocks the investigation and application of justice," said Antonio Morales Erlich, secretary general of the Christian Democrats. "I want to say as a lawyer the witness and evidence presented in the Romero case were ample to proceed with an arrest order."

Last Friday, the assembly, in its last session of the year and with only the ARENA deputies supporting the move, fired Giron Flores as attorney general, alleging "incompetence and immorality" and named ARENA sympathizer Roberto Garcia Alvarado to replace him.

"The truth is that I was fired for a simple and fundamental reason -- because the Romero case was about to be resolved," Giron Flores said. "Saravia was about to return and we were going to discover who was responsible . . . . The Romero case is now closed, and I was fired so the case could be closed."

Knowledgeable observers say Giron Flores is correct, because, by ruling the testimony was too old, although there is no Salvadoran law that limits the validity of testimony, any new evidence or testimony will be thrown out on the same grounds.

"On legal and procedural grounds, the case is dead," said one person familiar with the case.

Gregorio Rosa Chavez, auxiliary bishop of San Salvador, after a mass on Christmas Day, said, "In the church we think {Saravia} has valuable information on this act which convulsed the country."

ARENA presidential candidate Alfredo Cristiani said Giron Flores was fired for "demonstrable reasons showing he was not fit to continue in office" and accused the Christian Democrats of throwing up a "smokescreen" and "politicizing" the matter by trying to link the firing to the Romero case.

The closing of the case comes as right-wing death squads have again been appearing. On Monday, the Anticommunist Revolutionary Action of Extermination (ARDE) forced local radio and television stations to read a communique promising the "total extermination" of communists and listing prominent leftist leaders the group said it would kill.

Today, a group calling itself Anticommunist Hand of Central America (MACA) issued a statement saying "churchmen of the devil" and "communist diplomats" would be killed and said, "We assume the right to kill all communists." {A bomb destroyed the main office of the Lutheran Church in San Salvador Wednesday, The Associated Press reported. A leader said the church had received previous threats because of its work with war victims.}

The investigation into the Romero killing was being handled by the U.S.-trained Special Investigative Unit. According to two people involved in the investigation, Saravia was also to be charged with the 1985 kidnap and murder of a wealthy coffee grower, Carlos Guerra Campos, when he returned. Investigators said they planned to use the murder case to force Saravia to give evidence in the Romero case.

"It was to be our checkmate move," said one investigator.

Investigators also said Garay had identified Dr. Hector Antonio Regalado as the triggerman in the murder, although they said there was no hard evidence other than testimony linking the well-known dentist to the shooting.

Regalado, an expert marksman, was head of security at the Constituent Assembly when d'Aubuisson was president of that body in the early 1980s.

Earlier this year, two men who worked in the assembly at that time said Regalado ran right-wing death squads from the building.