The death of the three U.S. soldiers whose helicopter was shot down in El Salvador {front page, Jan. 3} is indeed sad. But it is crucial that Congress not take this as a reason for releasing the U.S. aid now being withheld from the Salvadoran government.

The condition Congress placed upon the release of the aid is that the Salvadoran government conduct an adequate investigation of the 1989 murder by the military of six Jesuit priests, their housekeeper and her daughter. This has not happened.

The current tragic incident raises further questions about investigation of human rights atrocities. Who, other than reporters, is questioning the witness who says that FMLN troops shot to death the two Americans who he said survived the helicopter crash?

A Jan. 7 news story reports that Silvio Mendez fears for his life because he has accused the FMLN rebels. Indeed, his ready pouring out of his tale to his neighbors and the press is surprising. But it is even more amazing that, as of five days after the Jan. 2 incident -- and two days after Mr. Mendez was first quoted in The Post -- neither U.S. nor Salvadoran officials have approached him for questioning. If he is telling the truth, why are they not protecting him? It is, of course, possible that they believe he is lying -- and it may better serve their purposes if they leave the questioning to the press.

How different this is from the handling of the witness to the murder of the six Jesuits and the two women who were killed by the Salvadoran military. Lucia Cerna was whisked to the United States with Ambassador William G. Walker, where she underwent severe, intimidating interrogation. While I am not suggesting that Mr. Mendez be put through a similarly menacing interrogation, clearly he should be questioned. And while they are at it, let's be sure the officials also question those witnesses who said the Americans died from lack of medical attention {front page, Jan 4}.

No matter how the U.S. soldiers died, however, their deaths are not reason for Congress to release the U.S. military aid to El Salvador. The Non-Government Human Rights Commission of El Salvador has reported that 115 civilians were killed in El Salvador in November of 1990. Although the commission was not able to ascribe 62 of the deaths to either side of the conflict, it linked 51 to the Salvadoran army and only 22 to the FMLN. Let's stop all aid to this government of proven brutality against its own people. SISTER CANICE JOHNSON Sisters of Mercy Silver Spring