ANCHORAGE, Alaska -- A serial killer who died in an Alaska jail after confessing to several murders began his crime spree with a sexual assault in Oregon, and the FBI is asking for help finding the victim.

Israel Keyes slashed one of his wrists and strangled himself with the bedding inside his jail cell, Alaska State Troopers reported Wednesday. Before he died, he confessed to killing Alaska barista Samantha Koenig and at least seven other people.

Background FBI: Alaska barista assaulted, dismembered

He also told FBI agents he raped a teenage girl in Oregon sometime between 1996 and 1998, and that she was the first victim in a twisted crime spree that ended with his arrest in March 2012 for Koenig's murder.

The sexual assault occurred on the Deschutes River near Maupin, Oregon, said the FBI's Beth Anne Steele. The victim was with friends and Keyes was able to discreetly separate her from the friends.

More: Alaska serial killer may have started in Oregon

Steele said there are no police reports that fit the scenario and the victim may not have reported the crime. If Keyes' confession is accurate, the woman would now be in her late 20s or early 30s. Investigators don't know whether the victim was from Oregon or was visiting the area for a vacation.

Keyes told investigators that he often times traveled great distances and liked to find his victims along hiking trails, at camp grounds and in other remote areas, Steele said.

Anyone with information on the sexual assault was asked to call the FBI in Portland at (503) 224-4181, in Bend at (541) 389-1202, or 1-800-CALL FBI from anywhere in the U.S.

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