The small, unarmed plane that crashed into a mountainside in El Salvador during a rainstorm Friday, killing four CIA employes, was following another plane, and CIA officials believe that it had locked its radar onto the other craft, according to federal officials familiar with the operation.

One official said it is not clear whether the use of the plane's radar to track the second aircraft contributed to the crash by depriving the CIA pilot of navigational information or whether the plane fell victim to the strong and unpredictable air currents associated with thunderstorms and with steep mountains.

A source familiar with the operation said there is no indication of what happened to the other airplane, which was believed to be headed for an airstrip used by antigovernment forces.

Washington Post correspondent Robert J. McCartney reported yesterday that the plane crashed on a volcano three miles from the capital of San Salvador, in territory controlled by the government, not on a second volcano 15 miles north whose slopes are a haven for the rebels, as U.S. Embassy officials originally said.

[The rebels' supply planes are believed to operate over distant eastern and northern provinces of the country, so it was unclear why a U.S. plane following a rebel craft would be flying so close to the capital, McCartney reported.]

For several months, the souurce said, CIA airplanes have engaged in regular nighttime surveillance using electronic sensing gear to search out small, low-flying craft headed for any of the 50 or more jungle airstrips used by the guerrillas.

U.S. military planes also conduct routine patrols over El Salvador, and Salvadoran military officials have said information from the reconnaisance flights is used in bombing suspected rebel positions.

The Salvadoran military has mounted a major offensive against guerrilla concentrations in the northeastern Morazan province, but it was unclear whether either of the planes was headed for that area. The crash site was more than 50 miles from the province.

The source refused to give details about the plane's equipment but said it allowed CIA pilots to follow targets "at some distance."

One congressional committee staff member said the CIA's patrol planes often operate without lights. The pilot was "one of the CIA's best down there, if not the best," another source said.

The deaths are the first known CIA casualties in El Salvador since the civil war began in 1979.

The CIA pilot had informed U.S. ground stations that he was following another aircraft, according to an official. Ground personnel were alerted to the crash by a radio beacon which emits a signal if a plane crashes, standard equipment on most aircraft.

State Department spokesman Brian Carlson yesterday released a statement saying, "The airplane's mission was to assist in locating and identifying shipments of arms and ammunition from Nicaragua to the guerrilla in El Salvador . . . . The airplane was operating under a program which had been fully authorized in accordance with the applicable procedures, including notification to the responsible committees of Congress."

Neither the State Department nor the CIA released the names of the four employes.

White House officials had no comment on the crash, which occurred two days before President Reagan's scheduled foreign-policy debate with Democratic presidential nominee Walter F. Mondale. The Democrats have repeatedly criticized the administration's covert operations in Central America.

Rep. Thomas J. Downey, (D-N.Y.) yesterday wrote CIA Director William J. Casey asking why the CIA was flying the mission.

A source familiar with the operation said that both military and intelligence officials fly surveillance missions but that the military aims at pinpointing the locations of rebel landing and docking sites, while the CIA operation aims at gathering information on landings and on distant supply deliveries.