The eight were arrested on Wednesday over an ambush that killed two Americans and an Indonesian, all teachers from a school run by PT Freeport Indonesia, a unit of U.S.-based Freeport-McMoRan Copper & Gold Inc., which operates mines in Papua's mountains.
"We are now searching for four more," police chief General Sutanto said after a meeting between President Susilo Bambang Yudhoyono and Patsy Spier, the wife of one of the slain Americans.
Sutanto did not say why police were specifically looking for four people, but four men detained along with the eight suspects last week have since been released.
Police dossiers against those eight were nearly complete and their trials would take place in Jakarta, Sutanto said.
"Statements from some suspects say all of them did it. But we need to get input from more witnesses, including from the Americans who have returned to the U.S.," Sutanto said, without elaborating.
The eight suspects could face the death penalty.
Spier, a survivor of the ambush, praised Indonesia's efforts to find the killers but insisted American agents should also continue to be involved.
"We discussed the importance of a transparent and credible process for this case. President Yudhoyono clearly shares this goal of transparency," she told reporters.
"I believe the process should include our FBI investigators participating in the interviews," said Spier, referring to the Federal Bureau of Investigation, which was involved in last week's arrests.
Indonesia-U.S. ties were strained by the incident.
Relations have improved and the arrests came after Washington restored military ties in November with the most populous Muslim nation as a reward for helping the U.S.-led war on terrorism.
One of the eight suspects is Anthonius Wamang, a separatist indicted by a U.S. federal grand jury in 2004 on two counts of murder and several counts of attempted murder over the killings.
The investigation into the shooting was a key factor behind the U.S. decision to restore military ties.
Before the resumption, Washington had said full military ties required prosecution of the people behind the Papua killings.
Washington cut back military ties after Indonesian troops shot demonstrators in East Timor in 1991, killing dozens, when the tiny territory was ruled by Jakarta.
Ties were severed after pro-Jakarta militias backed by elements in the military sacked East Timor in 1999 when the territory voted for independence.
Gen. Sutanto also denied persistent allegations that soldiers themselves were involved in the attack close to a massive U.S.-owned gold mine. The claims had complicated relations between Indonesia and the United States.
"Don't spread opinions like that, there is no proof," Sutanto told reporters.
Sutanto was speaking alongside Patsy Spier, the widow of one of the victims, after they both met with Indonesia's president.
Spier, who was injured in the attack, said she was in Indonesia to be interviewed by the police as a witness.
Last week, police arrested eight suspects in the killings on a windy and foggy road leading to the mine. One of the men, Anthonius Wamang, was indicted by a U.S. grand jury over his alleged role in the attack.
Sutanto said the attackers were all members of Papua's tiny separatist army, and that they intended to kill soldiers who patrol the road. He said police had forensic and other evidence, as well as confessions, to support the charges.
"Indonesian soldiers were in the first vehicle that went past, but they (the attackers) did not have time to shoot," he said."Then the second vehicle went past and they thought it also contained Indonesian soldiers."
Sutanto said the trials of the eight men would take place in Jakarta, and the men could face the death penalty for murder.
In a brief statement, Spier said the arrests were a"remarkable" development and praised the cooperation between the police and the FBI, which also took part in the investigation.
She declined to answer questions from reporters. (***)