(CNN) -- A suspected serial killer has killed for the last time.
Authorities say Israel Keyes, who was arrested and charged in the killing of an Alaskan barista, killed himself while in custody.
Before committing suicide on Sunday, Keyes confessed to at least seven other slayings, according to the FBI field office in Anchorage, Alaska, which on Monday asked for the public's help with tracing Keyes' travels over the years in the hopes of identifying any additional victims.
He crisscrossed the country, and authorities may never know how many he killed.
"Based upon investigation conducted following his arrest in March 2012, Israel Keyes is believed to have committed multiple kidnappings and murders across the country between 2001 and March 2012," the office said in a statement. "Keyes described significant planning and preparation for his murders, reflecting a meticulous and organized approach to his crimes."
Investigators are continuing to investigate those crimes, though they say Keyes confessed to killing at least seven other people besides the barista: two in Vermont, four in Washington state, and one somewhere on the East Coast, disposing of the body in New York.
Keyes did not know any of his victims, the FBI said, but looked for them at remote locations like parks, campgrounds and cemeteries.
He is said to have buried supplies he planned to use in future crimes, and investigators recovered two caches, one in Eagle River, Alaska, and one near Blake Falls Reservoir in New York. They contained weapons and items used to dispose of bodies, the FBI said.
Keyes allegedly confessed to killing Bill and Lorraine Currier of Essex Junction, Vermont.
He flew into Chicago, rented a car and drove across several states before arriving there, the FBI said. He is then thought to have traveled around the East Coast before returning to Chicago and then Alaska, where he had lived since 2007.
Prior to that, he lived in Washington state, where he confessed to killing four people, the FBI said. He is likewise alleged to have admitted to killing another person, somewhere on the East Coast, in 2009. The identities of those five victims were not released.
Finally, Keyes was accused and charged in the death of 18-year-old Samantha Koenig, the Alaskan barista. She was last seen on February 1 being led away by a man from the parking lot of the coffee stand where she worked. Her body was found in a lake north of Anchorage.
If he had been convicted in her death, Keyes could have faced the death penalty.