Stewart is indeed a very logical guess, but it's not him. There was a third man who chased after Booth but opened the wrong door. Joe, I do not believe he was a newspaperman.
Hint #1: I own over 70 Lincoln assassination books, and the answer (as far as I know) is in only one of them. It's in a book we've mentioned several times on the forum, and the book is not rare - most all assassination buffs would have it. The answer comes early in the book - in the section prior to chapter one.