U.S. officials 25 years ago supplied the names of thousands of members of the Indonesian Communist Party to the army in Jakarta, which at the time was hunting down the leftists and killing them in a crackdown branded as one of the century's worst massacres, former U.S. diplomats and CIA officials say.

For the first time, the officials are acknowledging that they systematically compiled comprehensive lists of communist operatives, from the top echelons down to village cadres in Indonesia, the world's fifth most populous nation. As many as 5,000 names were furnished over a period of months to the army there, and the Americans later checked off the names of those who had been killed or captured, according to the former U.S. officials.

The assassinations were part of a massive bloodletting that decimated the Partai Komunis Indonesia (PKI) in Indonesia, a Southeast Asian archipelago of more than 13,500 islands. The PKI was the third-largest Communist Party in the world at the time, with 3 million members, and through affiliated organizations -- such as labor and youth groups -- it claimed the loyalties of 17 million others.

Former high-ranking U.S. diplomats and Central Intelligence Agency officials described in lengthy interviews how they aided Indonesian army leader Gen. Suharto -- now president of Indonesia -- in his attack on the PKI, which left hundreds of thousands of people dead.

At the time, the United States was becoming involved in a war against communists in nearby Vietnam.

"It really was a big help to the army," said Robert J. Martens, a former member of the U.S. Embassy's political section who is now a consultant to the State Department. "They probably killed a lot of people, and I probably have a lot of blood on my hands, but that's not all bad. There's a time when you have to strike hard at a decisive moment."

White House and State Department spokesmen declined comment on the disclosures.

The former U.S. officials said that after they turned over the names to an Indonesian emissary, they became aware that Suharto's army was using the list to track down PKI members. Some said the lists were turned over with that purpose in mind. However, they were evasive when asked whether those picked up were being killed.

"No one cared, so long as they were communists, that they were being butchered," said Howard Federspiel, who in 1965 was the Indonesia expert at the State Department's bureau of intelligence and research. "No one was getting very worked up about it."

Former deputy CIA station chief Joseph Lazarsky and former diplomat Edward Masters -- Martens's boss -- said CIA employees contributed to the PKI lists.

But CIA spokesman Mark Mansfield said: "There is no substance to the allegation that the CIA was involved in the preparation and or distribution of a list that was used to track down and kill PKI members. It is simply not true."

It is not clear how many embassy officials knew that the names had been given to the Indonesian army beyond the small circle at the top. Lazarsky, when told that Martens had released the names of the communists, said: "That's news to me. I never heard that before."

Indonesian Embassy spokesman Makarim Wibisono said he had no knowledge of events described by former U.S. officials. "In terms of fighting the communists, as far as I'm concerned, the Indonesian people fought by themselves to eradicate the communists," he said.

The massacre was the culmination of decades of political tension between army leaders and Indonesian leftists. By the early 1960s, the PKI had taken a prominent role in Indonesia and the economy was on the brink of collapse, partly as a result of left-leaning, then-President Sukarno's lavish spending and his growing resistance to foreign aid from Western countries.

On Oct. 1, 1965, six anti-communist generals were assassinated in what is commonly referred to as an abortive "coup" against the army leadership. The military blamed the PKI and launched a lengthy crackdown on its perceived opponents, virtually eradicating the PKI.

Sukarno, who opposed the crackdown, attempted to restore order in the ensuing months but his leadership had been seriously eroded and he was forced to transfer key powers to Suharto in May 1966. Suharto became president in 1968 and has led the country since.

The assassinations of the generals were generally believed to have been carried out by rebellious army factions who, according to the present Indonesian government, were in league with the PKI. Whether the PKI was in fact involved in the assassinations of the generals, however, has been a matter of controversy among Asian scholars.

The number of alleged leftists and Chinese Indonesians killed in the bloodbath is also not clear. In 1966, The Washington Post published an estimate that a half-million were killed in the purge. In a 1968 report, the CIA estimated there had been 250,000 deaths and called the carnage "one of the worst mass murders of the 20th century."

Martens, an experienced analyst of communist affairs, said he headed an embassy group of State Department and Central Intelligence Agency officials that he said spent two years compiling the lists. He said he later delivered them to an army intermediary.

People named on the lists were captured in overwhelming numbers, Martens said, adding, "It's a big part of the reason the PKI has never come back."

Approval for release of the names came from the top U.S. Embassy officials, including former ambassador Marshall Green, his deputy Jack Lydman, and political section chief Masters, the three acknowledged in interviews.

Declassified embassy cables and State Department reports from early October 1965 -- before the names were turned over -- show that U.S. officials knew Suharto had begun rounding up PKI cadres and that the embassy had unconfirmed reports that firing squads were being formed to kill PKI prisoners.

The lists were a detailed who's who of the leadership of the PKI, Martens said. They included names of provincial, city and other local PKI committee members, and leaders of the "mass organizations," such as the PKI national labor federation, women's and youth groups.

"I know we had a lot more information {about the PKI} than the Indonesians themselves," Green said. "He {Martens} told me on a number of occasions that . . . the government did not have very good information on the communist setup, and he gave me the impression that this information was superior to anything they had."

The lists were turned over piecemeal, Martens said, beginning at the top of the communist organization. Martens said he supplied thousands of names to an Indonesian emissary over a number of months.

That emissary was an aide to Adam Malik, the Indonesian statesman, who was an ally of Suharto's in the attack on the communists. Interviewed in Jakarta, the aide, Tirta Kentjana Adhyatman, confirmed that he had met with Martens and received lists of thousands of names, which he in turn gave to Malik. Malik passed them on to Suharto's headquarters, he said.

U.S. Embassy officials then were said to have carefully recorded the subsequent destruction of the PKI organization. Using Martens's lists as a guide, they checked off names of captured and assassinated PKI leaders, tracking the steady dismantling of the party apparatus, former U.S. officials said.

Information about who had been captured and killed came to the Americans from Suharto's headquarters, according to Lazarsky. The Jakarta headquarters was the collection point for military reports from around the country detailing the capture and killing of PKI leaders, Lazarsky said. "We were getting a good account in Jakarta of who was being picked up," Lazarsky said. "The army had a 'shooting list' of about 4,000 or 5,000 people."

Lazarsky said the check-off work was also carried out at the CIA's intelligence directorate in Washington. By the end of January 1966, Lazarsky said, the checked-off names were so numerous the CIA analysts in Washington concluded the PKI leadership had been destroyed.