I've only ever spoken to one historian on that subject, and it was in brief. She was a specialist on the JFK conspiracies--which I was pretty amazed to find was an entire career's worth of work, but it was. She was tenured and everything.

Anyhow. She briefly mentioned the topic, and I've filled in the gaps myself to come up with a pretty definitive answer: there's no evidence to suggest there were any other killers involved in MLK's murder.

Again, that doesn't mean there weren't any. But there's no evidence to suggest otherwise. Ray was a known criminal with a lengthy arrest record, he was a vile racist, and he was in the right spot at the time of the killing. His rifle caliber matched the bullet wound, and his rifle had his fingerprints on it, and it had clearly been fired recently when it was discovered. He was almost assuredly the killer.

And there's nothing suggesting anyone else was involved. There's the famous Loyd Jowers case, but there was never anything to it but hearsay, and Jower's sister later admitted that he made the whole thing up so he could sell the story for $300,000.

Historians are not trained to speculate. Perhaps a cabal took out MLK and covered their tracks very well; perhaps there were simply a few extra bigots helping Ray plan his crime. But there has never been any evidence suggesting their presence.