While they look for more remains after finding the severed head, arm and leg of the Renton, Washington, nurse in a Seattle recycling bin last Saturday, local police are being assisted by the FBI in a wider search for possibly connected victims, PEOPLE confirms.
"This killing was a specific type of murder," says an FBI source who has knowledge of the investigation but was not authorized to speak publicly.
"Not to belabor the obvious, but it looks as if this guy planned it well in advance. It looks as if he targeted the victim, he groomed her for the kill. As an investigator, when you see a murder like this, you know you are dealing with a very sick mind."
"This could be a one-off murder, or it could be a serial killer."
Adds the source: "Our people are looking for other cases. I can't tell you any specifically. Did he do this before? Investigators are looking for unsolved cases with similar hallmarks."
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An attorney for Charlton, Gordon Hill, has not returned PEOPLE's calls
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Charlton told police that he and Lyne, 40, a divorced mother of three young girls, had been dating briefly after connecting online, and they spent last Friday night together before her family reported her missing the next day. He said he was too intoxicated to recall how they got to her home or what transpired there or afterward.
"At this time, the FBI is simply in communication with the Seattle Police Department," the FBI's Seattle spokesperson Ayn Dietrich-Williams tells PEOPLE. "We are assessing how we may be able to assist its homicide investigation."
Charlton has not entered a plea but is due in court April 27.
Reporting by SUSAN KEATING