Armed men in military uniforms were seen at the place where gunmen shot dead two Americans and an Indonesian teacher near the Freeport gold mine in Papua province, a report said Friday.

Agence France Presse quoted human rights investigator John Rumbiak as saying the information was revealed during an investigation he is conducting into the August 31 killings in coordination with Indonesian police.

Rumbiak, who heads Papua's Human Rights Study and Advocacy Institute (Elsham), said two witnesses alleged they saw people in military uniforms at the time of the shootings in which 12 other Freeport employees were wounded.

"They saw a number of people wearing military uniforms. They were on the roadsides at mile 62 and 63 when the incident happened. They were holding automatic guns," he was quoted as saying by AFP.

Police have recovered more than 100 cartridges from military ammunition at the scene of the crime on a road outside leading to Freeport's huge mine.

Rumbiak said he was working very closely with police and exchanging information.

The military insists the separatist Free Papua Organization (OPM) – which generally uses stone-age weapons – was responsible for the shootings.

But provincial police chief I Made Mangku Pastika has admitted that members of the Indonesian military might have been behind the fatal ambush. Rumbiak said police investigators who went to the crime scene a day after the killings were fired on by soldiers. "The Army said that they were there for protecting the incident site," he was quoted as saying by AFP.

The military had originally claimed that one armed Papuan man was shot dead and a soldier wounded during a clash on September 1 amid the hunt for the killers. But Rumbiak said an autopsy on the dead man revealed he had been quite sick and died 24 hours before the incident.