A key House Democrat yesterday accused U.S. officials in El Salvador of withholding relevant information from Salvadoran investigators about the killings last November of six Jesuit priests.

Rep. Joe Moakley (D-Mass.), chairman of the House Rules Committee and head of a special task force on El Salvador, said he had recently learned that the U.S. military adviser who implicated Col. Guillermo Benavides in the killings also signed a second affidavit saying senior Salvadoran officials may have known 10 days before the murders that Benavides had threatened the Jesuits.

Benavides was arrested in January, two months after the killings, shortly after the U.S. adviser told his superiors he had heard Benavides was involved. The colonel is being held pending trial.

Moakley said he was aware of the U.S. officer's first statement implicating Benavides, but that he was never told there was a second signed affidavit by the adviser, identified as Maj. Eric Buckland.

"If the information is accurate," Moakley said, "Salvadoran military authorities should have considered Col. Benavides the prime suspect immediately after the murders took place."

An administration official, who spoke on condition he not be identified, said Buckland's second affidavit was not given to Salvadoran officials because Buckland later recanted, saying he had been mistaken.

Moakley said yesterday he understood Buckland had recanted, but that it didn't matter "whether the account is true or false. My concern is, the substance of the account was not provided" to the Salvadorans until this week, and "only at my urgent request."

"At best, I consider this to be an unbelievable and inexcusable error in judgment," Moakley said, adding that Defense and State Department officials and the FBI had assured him they will "cooperate in determining why this occurred."

The administration official said a U.S. embassy official in El Salvador found a reference to Buckland's other affidavit two weeks ago while reviewing files in preparing for Buckland's testimony in San Salvador.

"We found it and brought it to the judge's attention. There was no effort made to conceal it," the official said. He added that the FBI had decided in January that Buckland's later statement was not credible.

"It is not correct" that Buckland's later affidavit was given to the judge as a result of Moakley's protests last week to the State Department, the official said. "When the embassy came upon this {Buckland's second affidavit} again," he said, "they felt it was something worth pursuing" and gave it to the judge.

Also yesterday, the United States and the Soviet Union issued a joint statement on El Salvador, calling on both sides to "intensify their negotiations with a view toward reaching political agreements and a ceasefire as quickly as possible."