Police believe foul play was involved in the death of a person whose partial remains were found Sunday by a group of environmentalists cleaning a lookout on Tantalus.

Honolulu Police Department Lt. Phillip Lavarias said a group of volunteers picking up trash on the steep hillside discovered the remains and called police at about 12:15 p.m.

He said the remains were in the advanced stage of decomposition and that the gender was undetermined.

Lavarias said foul play is suspected because of the circumstances, but could not provide further details. Police opened an unattended-death investigation.

At about 2:45 p.m. firefighters were called to the 3000 block of Tantalus Drive to help police and the Honolulu Medical Examiner’s office recover the remains. Recovery efforts were hampered by the terrain and rain, and firefighters remained at the scene for about three hours.

Isaak Janado, 23, of downtown Honolulu was with a group of four that was cleaning up several lookouts Sunday on Tantalus. Janado said the cleanup was organized by local clothing company Affinity Threads.

Janado, a company employee, said Cameron Calistro, the company’s co-founder, found a skull and called him over.

He said the bones were about 10 feet below the lookout in a spot extremely cluttered with trash and car parts.

Janado said the skull had long dark hair and teeth intact, but he could not determine the gender. He estimated the bones had been decomposing for months.

“We were just really, really scared,” he said. “I almost threw up.”

The group stopped cleaning and called police.

Calistro, 24, of Makiki, said he first noticed the skull under what appeared to be a pillowcase and then discovered a leg bone and a possible arm bone nearby. He didn’t see any torso bones.

The bones, he said, appeared to be from an adult. While the discovery was disturbing, he expressed hope it would help someone find a missing a family member or loved one.