An official investigative panel has found that mistakes by U.S. diplomats, particularly in probing a 1981 massacre of civilians in the El Salvadoran village of El Mozote, undercut the credibility of U.S. policy during El Salvador's 12-year civil war.

But the panel concluded that despite such lapses, U.S. diplomats generally worked hard to promote human rights in El Salvador during the turbulent 1980s.

These findings were contained in a 67-page study made public by the State Department yesterday. The study was ordered by Secretary of State Warren Christopher last March after a United Nations commission declared that thousands of murders and other abuses were committed in El Salvador during the 1980s, in some cases by leftist guerrillas but mostly by the armed forces and rightist "death squads."

The U.S. study was carried out by two retired career diplomats: George Vest, a former director general of the Foreign Service, and Richard W. Murphy, a former assistant secretary for Middle East affairs.

The panel found no evidence that department personnel in El Salvador or Washington sought to cover up the alleged involvement of Salvadoran security forces or death squads in a number of other much-publicized atrocities, including the separate 1980 murders of Archbishop Oscar Romero and four American Roman Catholic churchwomen, the 1981 killing of two American and one Salvadoran agrarian reform advisers and the murder in 1989 of six Jesuit priests, with their cook and her daughter, at the Central American University.

The panel's report noted that U.S. officials have been criticized widely for alleged lack of vigor in pursuing these cases, particularly the killing of the Jesuits. But it said examination of classified cables showed the United States pressed the Salvadoran government and armed forces leaders repeatedly to find the killers and bring them to justice, in some cases lending investigative assistance.

The report drew a mixed reaction from human rights activists and members of Congress who had opposed U.S. policy toward Central America in the 1980s. Many of these critics said they found the scope of the report too limited and, in some cases, too prone to justify State Department actions.

The report's basic finding was that U.S. ambassadors in El Salvador "consistently pushed their staffs to prepare honest, detailed human rights reports . . ., reporting officers pursued cases aggressively and the embassy put steady pressure on the Salvadoran government and military to bring perpetrators to justice."

Christopher specified that the investigation should not make judgments about former president Ronald Reagan's divisive policy of supporting the Salvadoran government and military against the guerrillas. But the report concluded that the controversy generated by the policy created "occasions when policy advocacy {by the administration} spilled over into statements that were perceived as misleading Congress or conveying 'disinformation.' "

It also said the Reagan administration was more interested in countering the criticism of human rights groups than in seeking ways to work together.

The report stressed that "mistakes were certainly made," particularly in the case of El Mozote where the U.N. report said more than 500 people, mostly women and children, were killed by the Salvadoran army during a sweep in guerrilla-held territory. When The Washington Post and the New York Times published accounts of the massacre in January 1982, administration officials sought to discredit them as a guerrilla propaganda ploy.

In an article in The Post last March 29, Thomas Enders, assistant secretary of state for inter-American affairs in 1981, denied U.S. officials had condoned or covered up the massacre. He said guerrilla activity around El Mozote prevented embassy officials from getting to the village to survey firsthand what had happened.

The report also said then-Secretary of State Alexander M. Haig Jr. had made "a clear mistake" when he told a congressional panel in 1981 that the nuns might have been killed inadvertently while trying to run a roadblock. The report said Haig's statement and remarks by U.N. Ambassador Jeane J. Kirkpatrick questioning the motivations of the nuns were widely interpreted as a sign the Reagan administration was callous about human rights.

"I think the report is a good first step," said Rep. Joe Moakley (D-Mass.) who headed a special House task force that looked into the killing of the Jesuits. Although Moakley is among those who criticized the Bush administration for not doing enough to pursue the Jesuit case, he praised Vest and Murphy for their work and a recommendation in the report to declassify the relevant cables. "The Pentagon and CIA should take similar steps," he said.

A much harsher verdict came from Sen. Patrick J. Leahy (D-Vt.), who said he felt the report "glosses over . . .the lies, half-truths and evasions that we came to expect from the State Department during that period."

"This report is sloppy, anemic and basically a whitewash," said Sen. Christopher J. Dodd (D-Conn.), chairman of the Senate Foreign Relations subcommittee on hemispheric affairs and long-time critic of Reagan's Central America policy.