The Indonesian government's decision to investigate anti-communist massacres in 1965 is being met with wariness by rights groups, some of which are reluctant to share information about mass graves until the government shows how it will conduct the probe.

President Joko "Jokowi" Widodo instructed security minister Luhut Pandjaitan this week to investigate the killings by the military and religious groups and gather information about mass graves. His directive followed a conference earlier this month that broke a half-century taboo on public discussion of an atrocity that historians say killed half a million people.

But Pandjaitan, a retired army general, has not instilled confidence that a genuine investigation will take place, saying the government would never apologize and that the death toll is wildly exaggerated, and demanding that advocacy groups prove that mass graves exist.

Haris Azhar of advocacy group Kontras said Wednesday he was involved in mapping 16 burial sites nearly a decade ago, mainly in central Java, and at the time had information about hundreds of other sites including on Bali, now Indonesia's top tourist island, and Sulawesi.

But he said for his group to share its information, the government must announce a clear and highly specific plan for the investigation. Thirty to 40 groups throughout Indonesia have information about graves, Azhar said.

"From our side, we decided not to give it to them if there's no clear agenda on what they will do with the data," he said.

The killings began in October 1965, shortly after an apparent failed coup in which six right-wing generals were killed. The dictator Suharto, a largely unknown major general at the time, filled the power vacuum and blamed the assassinations on Indonesia's Communist Party, which was then the largest outside the Soviet Union and China, with 3 million members.

Within Indonesia, widely accepted accounts of the era portray the events as a heroic uprising against communism and gloss over the deaths. Today, millions of descendants of Communist Party members remain stigmatized and face legal discrimination that prevents them from holding government jobs.

A four-year investigation released in 2012 by Indonesia's human rights commission described the killings as violence on a "truly massive scale" and called for the prosecution of perpetrators still living, but was ignored by the government of then President Susilo Bambang Yudhoyono.

Despite doubts over Pandjaitan's role, the government's tentative step toward a reckoning with one of the worst atrocities of the last century has been welcomed by rights groups and survivors.

"This is a very important step for the future of Indonesia," said Andreas Harsono, Indonesia researcher for Human Rights Watch. "It's just the opening battle."

Bedjo Untung, a survivor of the massacre and head of YPKP 65, or the Research Foundation for 1965 Murder Victims, said his group will soon hand over its information to the government.

"We already have evidence and records of mass graves in various places in Java and Sumatra," Bedjo said. "We will show evidence that an incredible crime against humanity occurred in 1965."