West German investigators said today they were examining the possibility that left-wing guerrillas murdered an American soldier to use his papers to smuggle a car bomb into the heavily guarded U.S. Rhein-Main Air Base in Frankfurt.

The explosion last Thursday killed two Americans and injured 20 other persons.

The military identity card of Edward F. Pimental, slain only hours before the bombing took place, was received in the mail today by a western news agency. The card was accompanied by an identical copy of an earlier letter claiming responsibility for the attack, and it was signed by the Red Army Faction and the French extremist group Direct Action.

West German police said the apparent connection could signal an ominous and more brutal change in tactics by the terrorist underground. It would mark the first time that a low-ranking U.S. soldier was targeted for his documents. Previous attacks have been directed mainly at prominent military or business figures or at NATO equipment sites.

Pimental, a 20-year-old specialist fourth class attached to the 563rd Ordnance Company in Wiesbaden, was last seen Wednesday night. He left a local discotheque around midnight with an unidentified man and a woman who reportedly made advances to him and suggested that they leave together, police said.

The soldier's body was found in a nearby forest the next day, shortly after the bombing at the air base. Pimental had been shot in the neck with a large-caliber weapon, with the kind of fragmenting bullets used in previous terrorist assaults, police officials said.

At the time, police said they could detect no immediate link between the bomb attack and the killing, which they surmised might have been perpetrated by a jealous boyfriend.

But the identical nature of today's letter claiming responsiblity for the bombing with that of the document sent to news organizations a day after the attack has led to another theory.

Some investigators believe that the terrorists may have killed Pimental for his identity card to foil the tight security at entrances to the Rhein-Main Air Base.

Police also say they have discerned strong similarities in the descriptions of the woman seen leaving the discotheque with Pimental and the woman who reportedly bought the car used in the bombing. The bomb was smuggled into the base in a green Volkswagen bearing American license plates stolen from another vehicle. The explosives were packed in gas cylinders, a technique used in previous terrorist bombings against a West German military school and the French Embassy in Bonn.

The letter bore the distinctive Red Army Faction symbol of a five-pointed star framing a machine gun. The Red Army Faction and Direct Action said they had attacked the American air base because it was a "center for war against the Third World." The two groups announced this year that they would conduct joint operations against "western imperialist targets."

Two incendiary bombs were discovered yesterday by cleaning women aboard a U.S. military train in Frankfurt used to transport troops in West Germany. Several blankets had been drenched in flammable liquid, but an ignition device had failed to spark a fire.

A police spokesman said today that the resurgence of terrorist activity indicated that the Red Army Faction may have gained new recruits and was prepared to carry out more brazen terrorist assaults.

But Interior Ministry antiterrorist specialists say that the Red Army Faction generally loses supporters when its attacks kill people. They say that the brutal killing of an American soldier could indicate a sense of desperation among the group's hard-core holdouts.

The investigation of the Rhein-Main bombings has concentrated so far on a dragnet for 12 suspected Red Army Faction members who are believed to have taken refuge among sympathizers in the Frankfurt area.