Two new suspects in the 1980 murder of four American churchwomen have been arrested by Salvadoran authorities, bringing the total number of suspects in custody to eight.

One of the new suspects, a former member of the Salvadoran National Guard who has become a born-again Christian, has confessed his direct participation in the crime, according to officials here involved in the investigation. All seven of the other jailed suspects are still members of the National Guard.

Officials here, who asked not to be named, outlined for the first time some of the basic evidence in the case and the sequence of events that led to the murder of three nuns and a lay worker on the night of Dec. 2, 1980.

While these officials said this evidence should be sufficient to convict six of the eight people now detained, they said a number of questions about the motives of the alleged killers and the possible role of their superiors remain unclear.

The officials would not reveal for publication the names of the two new suspects jailed late last month, nor would they say precisely how they were found, although the newly religious ex-guardsman appears to have undergone a crisis of conscience. He is now being held separately from the other suspects.

The new testimony is considered vital to the case. Physical evidence analyzed by U.S. experts last spring linked to the crime two of the six guardsmen who were first detained in April. A fingerprint on the churchwomen's burned-out van was identified as that of Sgt. Luis Antonio Aleman Colmindres, and a shell casing found at the scene of the killing was traced to the rifle issued to Cpl. Jose Roberto Moreno Canjura.

But until the new suspects were found, no participants in the crime had confessed, and Salvadoran law virtually requires a confession for conviction in a crime of this nature.

The extent to which there may have been a coverup after the murders was never seriously investigated, these officials said.

Salvadoran government officials have said repeatedly that they have made every attempt to bring the facts of the case to light. Diplomats who have followed the case closely have questioned at least the commitment of some members in the government to revealing the full facts and circumstances surrounding the murders.

One official here said privately last week that "the investigation was not focused on the question of whether there was a high-level effort to cover up."

One senior diplomat here, in contrast to assertions made by former U.S. ambassador Robert White, said he believes "there was never a deliberate high-level effort to cover up in the initial stages. But it strikes me that at some point as this went on there was something less than a total effort to get at the truth."

The investigation, at a virtual standstill last summer, received new impetus after Salvadoran officials visited Washington during the fall.

The Salvadoran government, meanwhile, still has not made public any of the findings of the special investigative commission appointed soon after the crime and headed by Commander of the Navy Col. Roberto Monterrosa.

The next stage in the case is expected to be the formal discharge from the National Guard of those suspects against whom there is believed to be enough evidence to go to trial.

Based on their reading of the testimony now at hand, much of which was made during lie detector tests conducted by a U.S. technician in January, officials here recount a series of events that began on the afternoon of Dec. 2, when Ursuline Sister Dorothy Kazel and lay worker Jean Donovan went to El Salvador International Airport to meet four Maryknoll nuns who were returning from a visit to Nicaragua.

"What touched the whole thing off, what provoked the crime, was a phone call from one of the guardsmen at the airport," one official said here this week. "Something made him uneasy."

According to this official, the soldier who was so mysteriously suspicious of Kazel and Donovan called his immediate superior in the Salvadoran National Guard, Sgt. Aleman Colmindres, to voice his reservations.

Yet, when only two of the Maryknoll sisters arrived on the midafternoon flight from Managua, Kazel and Donovan picked them up and left the airport without any problem.

Kazel and Donovan returned to the airport a few hours later to meet Maryknoll sisters Ita Ford and Maura Clarke who had caught a later flight. It was now dark.

"On the second trip something happened and we still don't know exactly what it was," said one official close to the investigation.

The guardsman who called Sgt. Aleman Colmindres has made a statement, but is not one of those detained because he was not involved in committing the crime, officials said.

This account of what precipitated the killings contrasts sharply with bits of publicly reported evidence from other sources suggesting that the murders may have been ordered at a much higher level and that the return of Clarke and Ford was being awaited by their killers.

Former ambassador White has said that several weeks after the killings, a "high ranking Christian Democrat in the El Salvador government" told him a radio transmission between military units near the airport on the afternoon of Dec. 2 contained the words, "No, she didn't arrive on that flight. We'll have to wait for the next."

Sister Marie Rieckelman, who was on the early flight and continued to Miami, has told reporters in the United States that Salvadoran soldiers boarded the plane, scrutinized her closely and instructed the stewardesses to question her twice about where she was going.

There are also several reports that the Salvadoran military suspected Ita Ford of aiding guerrillas in the northern province of Chalatenango where she lived.

Officials here insist, however, that any leads indicating an order from on high to kill any or all of the churchwomen have been thoroughly investigated and none could be substantiated.