Specks of unexplained DNA were found on the holdall containing the curled-up body of MI6 spy Gareth Williams, his inquest has heard.
Det Ch Insp Jackie Sebire said "two minor components of another contributor's DNA" were found on the zip toggle and padlock.
She said it was her opinion "that a third party had been involved".
Mr Williams's death has puzzled Scotland Yard detectives who have been unable to determine a cause of death or establish whether he had locked himself into the bag.
But Det Ch Insp Sebire, who is leading the investigation, said: "My thought or my opinion since I went into the scene is that a third party had been involved in the death or by putting the body in the bag."
The fact the DNA had been found meant there remained "future work that needs to be done," she said.
But she added the vast majority had already been carried out and there was "limited scope" for further forensic discoveries.
Westminster Coroner's Court was shown a video of what detectives saw as they arrived at his flat on 23 August 2010.
There was a cutting from the Observer newspaper about the top five regrets of the dying, make-up and a red wig...read more