The CIA plane that crashed here four days ago was based at San Salvador's main Air Force base and had been flying night missions for several months, airport sources said today.

The plane was a silver-colored Merlin twin-engine turboprop, according to the sources, who are employed at the airport in the Salvadoran capital's suburb of Ilopango. The base is about 12 miles east of where the plane crashed on the San Salvador Volcano during a storm before dawn Friday.

One civilian source at the airport said military personnel there had told him that the plane contained "a reconnaissance laboratory."

The source said he did not know of any other planes of the same type at the airport, but he said that he had seen the one that crashed taking off several times in the late afternoon in recent months.

"It mostly flew at night," the source said.

Merlins, which are built by Fairchild, can carry eight or more persons. The U.S. government has said that the plane was carrying four passengers, that all were killed in the crash, and that they were U.S. citizens employed by the CIA. Their names have not been released.

U.S. officials in Washington have said that the plane carried sophisticated reconnaissance equipment and that it was monitoring activities of left-wing insurgents. In particular, they said, it was seeking evidence of Nicaraguan supply shipments to El Salvador's left-wing guerrillas.

Before the plane's crash, it had not been stated publicly that CIA planes were carrying out reconnaissance missions over El Salvador.

U.S. Army reconnaissance planes based in Honduras have been flying missions over El Salvador since early this year, and U.S. military planes based in Panama have been doing so for longer than that.

There have been a number of discrepancies in various U.S. accounts of the plane and its activities. Immediately after the crash, intelligence officials in Washington suggested that the plane had taken off from a U.S. installation at Palmerola, Honduras.

At the same time, U.S. officials here told at least four reporters in separate conversations Friday that the plane had crashed on the Guazapa Volcano, 15 miles north of the capital, rather than on San Salvador Volcano, just two miles outside of the city.

Guazapa's slopes normally are controlled by the guerrillas, while the San Salvador Volcano is firmly in government hands.

On Saturday, U.S. government sources also said that the plane had been following another plane at the time that it crashed, and that the plane being followed was believed to be heading for an airstrip used by the guerrillas.

This description of the incident suggested that the CIA plane had caught a plane from Nicaragua delivering supplies to the guerrillas.

But one well-placed source here, who insisted on remaining anonymous, said he doubted that the CIA plane was chasing another one. He thought that it was more likely that the CIA plane simply was taking off or returning from a routine reconnaissance mission.

The Salvadoran Air Force threw up a security cordon around the plane's wreckage on the San Salvador Volcano's slopes Friday and has not permitted journalists to visit the crash site. Military authorities also barred airplane rental companies from taking up journalists to fly over the volcano.

U.S. officials say that there have been numerous reports of unidentified small planes traveling between Nicaragua and guerrilla-dominated areas in eastern El Salvador, and they have suggested that these were covert arms flights.

But the officials acknowledge that none of these flights ever has been intercepted, and they have declined repeated requests from journalists to identify locations of any airstrips allegedly used by the rebels.