“From the inquest, we set up our investigation in a different way, and this has led to greater clarity on some aspects of the case,” Deputy Assistant Commissioner Martin Hewitt, who oversaw the police investigation, said in a statement. “Now, at the end of our investigation, based on the evidence, or where we have been unable to find positive evidence, we believe that it is a more probable conclusion that there was no other person present when Gareth died.”

After three years of investigation, Mr. Hewitt said, “many questions remain unanswered.”

He said there was no evidence that Mr. Williams’s apartment on Alderney Street in the Pimlico neighborhood — close to the headquarters of the Secret Intelligence Service, where he had been working — had been “deep-cleaned,” a term used for the removal of forensic traces by intelligence operatives. There was also nothing to suggest a struggle or a break-in, he said.

Though the police did not themselves demonstrate that it was possible for a person to lock himself inside a bag, Mr. Hewitt said, they saw video of this being done.

Why Mr. Williams would have done so was not entirely clear. The 2012 coroner’s inquest heard evidence that he had visited websites dedicated to bondage and claustrophilia, which involves seeking sexual thrills from being shut in enclosed spaces.