FAIRPLAY, Colo. -- A 24-year-old Colorado Geological Survey intern, whose body was found lying in a shallow creek in a remote part of Park County Tuesday night, was strangled to death, an autopsy revealed Friday.

A camper is accused of killing Alyssa Morimoto after she radioed her geology instructor and screamed for help. Her body was found 10 hours after the help call came in.

Robert Amos, who is also known as Dennis Cook or Dennis Lee Cook, faces first-degree murder charges and is being held on $800,000 bond.

Her body was found with a gray and green strap wrapped around her neck, according to the arrest affidavit.

Deputies, who had been sent out to search for Morimoto, found drag marks from Amos' campsite leading to the gray Jeep, then toward the creek, the affidavit said.

The Park County coroner, Sharon Morris, said Morimoto died from being strangled with a belt ligature.

Morimoto and her instructor were mapping an area in the San Isabel National Forest Tuesday and had decided to separate earlier in the day so they could survey different areas. Morimoto took the Jeep and parked at the dead end of U.S. Forest Service Road 434, not far from Amos' campsite, authorities said.

Morimoto radioed her instructor, Karen Houck, that she spoke with "some camper" and received permission to park the vehicle near his campsite, Houck said. About 20 minutes later, she screamed, "Karen, help! Help! Help!" There was another radio transmission with just her screaming for her life, Houck told officers.

After logging onto Morimoto's GPS signal, Houck hurried to find her and was met by Amos who was jogging toward her near the Jeep.

At Houck's request, Amos gave her a ride so she could get in range of cell phone service to call authorities.

Shortly after, a forest ranger was spotted driving up the road and Houck flagged her down to call the Park County Sheriff's Office. Amos was questioned and later arrested in connection with Morimoto's death.

He told Park County authorities that he had left his campsite Tuesday morning at about 9 a.m. or 9:30 a.m. and did not return until he saw Houck. This contradicts Houck's statement that Heberton-Morimoto had called to say that she had spoke to "some camper" before she parked the Jeep next to his campsite.

Amos, 44, had served 19 years in prison for a 1981 murder in Kansas, state records said.

Amos was convicted in a 1981 murder in Kansas. He was sentenced in 1982 to 15 years to life in prison.

He served a total of 19 years in Colorado and was also convicted of assault while in prison, according to Department of Corrections spokeswoman Katherine Sanguinetti. After he was released in 2001 for parole, he served probation in Kansas for another two years.

In 2003, he was officially released from Colorado's correctional system having served at four facilities in Buena Vista, Centennial, Colorado State Penitentiary, and San Carlos State Penitentiary in Pueblo.

Anyone with information about the case is asked to call the Park County Sheriff's Office at 719-836-2494.