SAN SALVADOR, FEB. 15 -- The Salvadoran Army captured an important cache of guerrilla documents and may have killed one of the top five rebel commanders in a firefight, military intelligence sources said today.

Col. Juan Orlando Zepeda, head of military intelligence, said the documents outline measures to unify the five armies of the the Marxist-led Farabundo Marti National Liberation Front (FMLN), a strategic analysis of the war and options for 1988, and an evaluation of the work with the civilian population.

"The documents are very significant and are of the highest level, of that we are sure," Zepeda said. "There are letters from the top commanders and plans, and we will know more when we have had time to analyze them."

Besides the intelligence coup of the documents, the death of a top commander, if true, would be a major blow to the rebels. Officers were weighing evidence that the dead man was Ferman Cienfuegos, 40, head of the Armed Forces of National Resistance, one of the five FMLN groups.

The military began a serious investigation into the identity of a dead guerrilla only after reporters, hearing rumors of the death of a commander from sources linked to the rebels, began to inquire.

Soldiers involved in the fighting were flown to the capital to tell their story to military intelligence and two reporters were allowed to attend the meeting. According to the platoon leader, still in combat dress, the firefight took place Wednesday in Chalatenango province.

When following the trails of the blood of wounded guerrillas after a brief firefight with about 20 insurgents, three soldiers came on a badly wounded rebel and two back packs containing letters and communications from the highest level of the guerrilla command.

The soldiers who found the rebel said he was wounded at least three times in the stomach, one leg was shot off, and his head was bloody. One soldier said he killed the rebel when it was obvious he was not going to live long, and they then burned the body, as ordered to do with dead guerrillas.

The soldiers' description of the man was close to that of Cienfuegos, known for his thin, scraggly beard and dark, plastic-framed glasses.

In a statement released in Mexico City, the FMLN said the report was false and that Cienfuegos was "fulfilling his functions in the people's war. This is a manuever designed to hide the low morale and defeat of the puppet Army."

Three soldiers involved in the action independently picked out Cienfuegos as the man killed when shown pictures of the commander surrounded by other rebel leaders.

Officers showed that a pair of glasses like Cienfuegos' were found in a pack, along with a special shortwave radio, and much of the correspondence was addressed to Cienfuegos, whose real name is Jose Eduardo Sancho Castaneda.

But the intelligence sources were cautious, saying that despite the physical resemblance, the dead man may have been someone carrying Cienfuegos' equipment and documents, and that the guerrillas probably would have launched a serious attack to try to recover the body if it was one of their top leaders.

According to sources analyzing the documents, many still in undeciphered code, they give great detail on groups operating in the capital, the guerrillas' relationship with various labor unions and other groups, and detailed plans to escalate violence in an attempt to provoke the military into violent repression.

Military observers said they are constantly amazed at the amount of information the FMLN writes down. "When they are not fighting, they are writing," said one experienced military observer.