A man who turned himself in and said he killed two convicted child rapists might have been motivated by a notorious case in Idaho in which two children were abducted in mid-May.

Michael Anthony Mullen (search) called police Monday to turn himself in and later confessed to killing Hank Eisses (search) and Victor Vasquez (search) on Aug. 27, officers said.

"One possible reason was the case in Idaho," police Lt. Craige Ambrose told The Associated Press. Ambrose wouldn't elaborate, saying the case was still under investigation.

Mullen was booked into jail for investigation of murder pending a court appearance.

Police said they believe Mullen's claim that he killed the two registered sex offenders because he knew details only the killer would know. He knew that the victims were each shot once in the head, and he knew the caliber of the weapon.

"Mullen also said that he had planned the murders for some time and that on July 13, 2005, he had accessed the Whatcom County Sheriff's sex offender Web site, and from that selected at least one of the two victims," according to a police department news release.

As is typical in Washington, the sheriff's Web site lists the residences of sex offenders who are required to register with local authorities.

Mullen, 36, has a criminal record but no history of violence, Ambrose said.

In the Idaho case, Joseph Edward Duncan (search) is accused of fatally beating three people with a hammer in a home outside Coeur d'Alene, Idaho, in order to abduct two children for sex.

He was arrested July 2 at a restaurant with one of the children, 8-year-old Shasta Groene. Remains of her 9-year-old brother, Dylan, were found in a Montana forest two days later.

According to police, Mullen said he mailed a letter about the Bellingham killings to several news outlets last week. The Bellingham Herald, the daily newspaper in this college and mill town of about 71,000, reported Thursday that the letter threatened other sexual predators.

"He said there are more letters forthcoming," Ambrose said of Mullen's confession.

Eisses, 49, and Vasquez, 68, were found shot to death at their apartment Aug. 27. They were both classified as Level 3 sex offenders, the type considered most likely to re-offend.

Their bodies were found by a roommate, also a sex offender, who said a man wearing a blue jumpsuit and a cap that said "FBI" came to their apartment, told them he was an FBI agent and said he wanted to discuss their status as sex offenders.

The supposed federal agent told them that one of them was on a "hit list," police said.

The roommate said he left while the FBI impostor was still there and he found the bodies when he returned about four hours later.

Vasquez was convicted in 1991 of molesting several relatives, according to court documents. He was released from prison about two years and remained under Department of Corrections supervision.

Eisses was sentenced to 5½ years in prison in 1997 for raping a 13-year-old boy. He was released from supervision about two years ago, said Kit Bail, Corrections Department field supervisor for Whatcom County.