Three members of Congress abruptly canceled an unofficial visit to El Salvador in January after the American Embassy in Mexico City warned that one of them had been targeted for a clandestine "investigation" by a Salvadoran "crony" of Robert D'Aubuisson, the right-wing president of El Salvador's constituent assembly.

The warning was issued to Rep. George Miller (D-Calif.) who, according to a State Department cable, was singled out by D'Aubuisson as someone whose liberal views were a threat to the Salvadoran regime and whose activites in the United States should be investigated by a retired Salvadoran security agent.

Miller was accompanied on the Jan. 9 trip by Rep. F. James Sensenbrenner (R-Wis.) and Rep. Les AuCoin (D-Ore.).

Reached last night, Miller said, "I've had my free speech chilled."

Miller said he has expressed his concern about the episode to the State Department and the FBI. He said he has been frustrated in attempts to get a full explanation from U.S. officials on the scope of D'Aubuisson's plans to investigate him.

The congressmen were scheduled to visit Mexico, Guatemala and El Salvador with members of an activist church group. A standard "threat assessment" was given by the State Department before the group left Washington. It issued what reportedly is a routine finding that the security of the group could not be guaranteed.

When the group reached Mexico City, however, embassy officials issued a much stronger warning to Miller, and a cable from the embassy in San Salvador was shown to Miller. The cable contained a reference to Miller's April, 1982, visit to El Salvador with House Majority Leader James C. Wright Jr. (D-Tex.) and the negative reaction by right-wing government officials to Miller's views.

"The embassy discovered that a D'Aubuisson crony and security man, retired Col. Ramone Suvillaga, traveled to San Francisco to 'investigate' Miller following the visit," the cable said. As a result of the warning, the congressmen skipped their planned visit to El Salvador.