Ok, I think we are getting real close. I have an issue with the order of clauses in the first line. The current wording gives too much weight to Oswald being the assasin (as a firm fact) vs being identified/found as the assassin. Compare to Ruby as the killer of Oswald that was caught on film (redhanded)

Lee Harvey Oswald (1939-10-18 – 1963-11-24) was the assassin of US President John F. Kennedy according to four US government investigations into the assassination.

How about  

Lee Harvey Oswald (1939-10-18 – 1963-11-24) was, according to four US government investigations, the assassin of US President John F. Kennedy 

 or

According to four US government investigations, Lee Harvey Oswald (1939-10-18 – 1963-11-24) was the assassin of US President John F. Kennedy.