There is a show on Travel Channel called Legend Hunter. The host did a very detailed and interesting investigation and it looks like the killer was a American doctor (can’t remember his name) who Scotland Yard had in custody as a suspect and jumped on a ship back to America when released on bail. They matched his writing and signature to the letters that were sent to the police at the time. It is worth a watch if you see the show.
That would be Dr. Tumblety, who was in London during the murders, arrested, then left for New York.
Sounds like you are talking about Francis Tumblety. I don’t think it’s very likely he was the Ripper, knowing what we know about the psychology of serial killers nowadays. Tumblety was a homosexual, after all, and the Ripper killed exclusively female victims in typical sadistic sexual murders. Psychopaths generally pick victims based on their sexual preferences, so a male homosexual psychopath will pick male victims (like Jeffrey Dahmer, or John Wayne Gacy), while a male heterosexual psychopath picks female victims (like Ted Bundy).