Telly Nathalia, Jakarta – Indonesian police have questioned 19 soldiers as witnesses in an investigation into the killing of two American school teachers and an Indonesian last month in rebellious Papua province, police said on Sunday.

The only other witness was an indigenous Papuan who police said once worked as a guide for the feared Kopassus special forces and who claimed he knew who carried out the attack.

Separately, one Indonesian security source who declined to be identified said the probe so far indicated Papuan rebels were not behind the ambush – which the military and some officials have blamed on the separatists or one of their splinter factions.

Papua deputy police chief Rajiman Tarigan told Reuters that the civilian witness, who was not under police protection, was in the general area at the time of the August 31 shooting near the world's largest copper and gold mine, operated by US-based Freeport-McMoran Copper and Gold Inc.

Asked if that witness said he knew who did it, Tarigan said: "Yes, that was his information." He declined to give details.

Papua police spokesman Yosep Iswanto said nearly all the witnesses to have been probed were soldiers "because they were assigned in that area. Their duty was to safeguard that area".

The military provides the main security for Freeport's mine, and the killings took place on a road not far from an army post. Iswanto said it was possible that any of the 20 witnesses who had been investigated in the last month could become suspects.

The United States has called the ambush by an estimated 15 gunmen on a convoy of vehicles carrying mainly American school teachers and their families an act of terrorism.

Both the Free Papua Movement (OPM), which has been fighting a low-level rebellion for decades in the remote eastern province, and the military have vigorously denied any involvement in one of the worst attacks on foreigners in Indonesia in years.

The security source said the probe indicated OPM had not pulled off the ambush, partly because of the amount of ammunition used. Experts on Papua say the rebels are generally poorly armed. "If we look at OPM, it's not possible, because from the assault some 200 bullets hit the vehicles," said the source.

Iswanto said the soldiers who had been investigated came from Battalion 515 of the army's strategic reserve. He said about 65 percent of the investigation had been completed.

Some separatist supporters and human rights groups have claimed army elements might have staged the incident to discredit the rebels, charges vehemently denied by the military.

One foreign expert on Papua said the provincial police chief, Made Mangku Pastika, who has previously won praise for attempts to handle Papua's political problems sensitively, was conducting a serious investigation into the killings.

Papuans differ ethnically from most people in the world's most populous Muslim country and are either Christian or animist.

Indonesia's military has a murky reputation, having played a key political role during the rule of former autocrat Suharto, whose downfall in 1998 sparked calls for investigations into allegations of rights abuses carried out by the armed forces.

In a further blow to the military's image, a number of Kopassus special forces troops have been declared suspects in the murder last November of Papua's top independence leader, Theys Eluay, who advocated separation through peaceful means.

Papua is one of Indonesia's two separatist hotspots. The other is Aceh on the northern tip of Sumatra island.