Repeated gunshots were heard seven hours before the blasts on Thursday night at a road in Timika, Papua, leading to a massive mine operated by Freeport-McMoRan Copper & Gold Inc., the police said.

"There were repeated gunshots seven hours before the bomb exploded, but the authorities never found those who fired them, "said Insp. Gen. Abubakar Nataprawira in a press conference in Jakarta on Friday.

He said another round of gunshots were heard a month before around the same area that leads to the Grasberg mining complex, which is one of the world's largest single producer of copper and gold.

Abubakar said the blast happened at about 1 p.m. and that the type of explosive used was of an older generation of mortar typically used during the war in Papua. He did not specify which war.

"There were actually two mortars, placed on a burning stove. Only one of those mortars exploded," he said explaining the mechanism used by the perpetrator.

According to The Associated Press more than a dozen members of the country's elite anti-terrorism unit arrived at the scene. "An unidentified group was trying to attack Freeport's facilities," said military Lt. Col. Tri Suseno.

Papua is home to separatist rebels who have long denounced the mine as a symbol of Jakarta's rule over the region. Rebels were blamed for a 2004 attack on the same road that left two Americans dead.

Indonesian security forces hired by Freeport to guard the mine were initially suspected of taking part in those killings to extort higher protection payments from the New Orleans-based company.

The mine has seen violent worker protests in the past, and environmental groups accuse the company of pollution and stripping the desperately poor province of its natural resources.

Indonesia tightly controls journalists' access to the province and Freeport routinely turns down requests to visit the sprawling mine.