Russell Armstrong's Family -- He was MURDERED

Russell Armstrong's family isn't buying suicide as the cause of death ... they believe he was murdered, and they're putting a private dick on the case.

Although the L.A. County Coroner ruled Russell's death was a suicide by hanging, sources close to the situation say the family is convinced ... Russell's money problems caused him to get involved "with the wrong people." They believe Russell may have screwed someone over out of desperation and that person or persons retaliated by killing him.

We're told the family is in the process of hiring a private investigator.