A U.S. Federal Bureau of Investigation laboratory report has linked two Salvadoran National Guardsmen to information supplied by the government here in the December killing of four American churchwomen, according to sources close to the controversial investigation.

The two soldiers are among six guardsmen held since April 29 as suspects in the case.

A summary of the report was shown to Salvadoran officials last week, and they have just received the full text. So far no official announcement of conclusions drawn from the report has been made. A spokesman for the U.S. Embassy here declined to comment on the evidence.

The report said that the fingerprints of one of the six arrested guardsmen, Sgt. Luis Antonio Colmindres Aleman, matched a single fingerprint that the Salvadoran government said it found among many on the van in which the churchwomen were traveling on the night of Dec. 2 when they were killed.

A separate summary in the report said that FBI balistics tests on shell casings the government said were found by its investigators at the scene of the deaths matched a military-issue, West German-made G3 rifle the Salvadoran government said had been issued to Cpl. Jose Roberto Moreno Canjura.

The fingerprints and the rifles of the six arrested guardsmen, whose names were supplied to the U.S. Embassy by a confidential informant, had been sent to Washington for tests and analysis several weeks ago.

[Sources in Washington familiam with the investigation confirmed that one of the six sets of prints matched those the Salvadorn government earlier had said were found on the van, and one of the rifles matched one of the shell casings also sent by the Salvadorans for FBI analysis. These U.S. sources stressed, however, that the FBI had reached no conclusions based on either set of information concerning the circumstances under which it had been gathered or what it proved about the guilt of the parites in question beyond the actual mathch-ups.]

Politically this case is extremely sensitive. The killing of the three nuns and one religious lay worker provoked a brief suspension of aid vital to the Salvadoran government in December. Church groups, especially in the United States, have cited the slowness of the investigation as one reason the United States should withdraw completely its support of the military-civilian coalation government here.

But the investigations, arrest and prosecution of soldiers is regarded by many Salvadoran officers, including some at the highest levels of the government, as dangerous to the morale of their troops fighting a persistent leftist insurgency.

Maryknoll nuns Ita Ford, 40, and Maura Clark, 49, Ursuline Sister Dorothy Kazel, 41, and lay religious worker Jean Dovovan, 27, were shot to death and at least two of them apparently were raped after leaving the international airport on the Pacific coast about an hour away from the Capital.

At least four of the six detained guardsmen allegedly were assigned to duty near the airport that night, including Colmindres, according to informed sources. There is some information, according to one source involved with the investigations, that Moreno Canjura allegedly joined the group elsewhere.

Information that could point to the direct involvement of the remaining four guardsmen was not established by the FBI report, sources said.

But even if the government succeeds in linking Colmindres and Moreno Canjura to the killings based on the FBI technical report, sources said, that evidenced may be sufficient to convict them under Salvadoran law. The judicial system here requires, as one analyst put it, "practically eyewitnesses in broad daylight at high noon" to obtain a conviction. And while witnesses are required there are virtually no incentives allowed under Salvadoran law for participants in the crime to turn state's evicence.

Adding to the problem, intimidation and assassination of judges and attorneys working on politically significant cases are commonplace.

In local terms, the case against the guardsmen remains weak and a number of questions remain unresolved. The luggage carried by Ford and Clark, who returned here from Nicaragua the night they died, has not been found. Neither has the money carried by any of the four, according to informed sources.

There is also a lack of any clearout motive for the killings. Although several priests, including the archbishop of San Salvador, has been assassinated since 1977, the murder of American religious workers or nuns was unprecedented.

All four worked actively with El Salvador's poor and had frequent confrontations with military officials in their areas. But no link has been established prior to the night of the murder between the women and any of the suspects, according to sources connected with the investigation.

There has been speculation that the deaths may have been ordered by some of the soldiers' superiors. At the time, Christian Democratic Party leaders, including junta member Jose Napoleon Duarte, described the murders as attempts by rightist extremists, possibly within the government, to destabilize the fragile junta. But no evidence has appeared to substantiate such theories, sources said.