William Pepper and Laurie Dusek say Sirhan did not kill Kennedy but have ruled out a security guard long suspected of playing a role in the slaying at the former Ambassador Hotel, CNN reported.
The attorneys, who say two guns were fired in the assassination, want a new trial for Sirhan, 67, who is serving a life sentence in California for the shooting. They've argued Sirhan was hypnotically influenced to fire shots at the senator as a diversion.
In a federal court filing last month, the New York attorneys said Thane Eugene Cesar, a security guard escorting Kennedy at the time he was shot, is not the real killer.
"It is my personal belief, at this time, that the security guard, Cesar, was not the second shooter," William Pepper said in e-mail to CNN.
Pepper alleges Cesar, however, may have been involved in an assassination conspiracy.
"We need an evidentiary hearing to deal with the second shooter and his identity," Pepper told CNN.
The defense attorneys say none of Sirhan's eight shots hit Kennedy. They argue the senator was struck by shots fired by a second gunman positioned behind the candidate, CNN said.