Los Angeles (CNN) -- If there was a second gunman in Sen. Robert F. Kennedy's assassination, who was it?

Lawyers for convicted assassin Sirhan Sirhan claim their client did not fire any of the gunshots that struck the presidential candidate in 1968. And in their latest federal court filing, they also rule out another man some have considered a suspect -- a private security guard named Thane Eugene Cesar, who was escorting Kennedy at the time he was shot.

Attorneys William Pepper and Laurie Dusek insist someone other than their client, Sirhan, fatally shot Kennedy. They now say the real killer was not Cesar, a part-time uniformed officer long suspected by some conspiracy theorists of playing a sinister role in the senator's murder.

Pepper and Dusek made the claim in papers submitted to a U.S. District Court in Los Angeles late last month....