A body is removed from a Surrey condominium complex in October 2007. (CBC)

An RCMP officer involved in the investigation of the slaying of six people in Surrey, B.C., in 2007 has been reassigned because he allegedly got involved in a relationship with a witness, according to a news release from the force late Tuesday.

The officer taken off the investigation is a sergeant in his 40s, who is "well respected" and a "good cop," police sources told CBC News.

'This person has put a dent in our case.'—Eileen Mohan, mother of victim Chris Mohan

A total of six alleged gangsters were charged in the mass killing. One of them, Dennis Karbovanec, has since pleaded guilty to second-degree murder and is serving a life sentence.

Earlier reports that the witness the RCMP sergeant was involved with was an ex-girlfriend of Karbovanec were flatly denied by RCMP.

The Mounties did say the officer is alleged to have had "an unprofessional relationship with a witness in an ongoing investigation." He now is the subject of an internal police investigation and has been assigned to administrative duties pending the outcome.

Eileen Mohan — mother of Christopher Mohan, one of two men who were unintended victims in the slaying — said she was worried the alleged actions of the officer will taint the case against the others accused in her son's death.

"This person has put a dent in our case," Mohan told CBC News.

"He's put a question mark there. It will only give motivation and ammunition to the defence counsel to use this incident.

"You have to recognize that six lives were lost that day and if this [RCMP officer] didn't feel the pain of all the families then I don't know. He should not be on the team at all."

Any information the witness may have provided has not been used in any legal proceeding so would not compromise the outcome of the Surrey Six case, the RCMP statement said.