There was definitely more than one shooter, based on the different trajectories of the wounds in JFK's body (or, alternatively, you could buy the bouncing bullet theory). The forensics the FBI used to determine that there could only be one shooter was disproved in 2003 so thoroughly that the FBI no longer uses it (http://www.washingtonpost.com/wp-dyn/content/article/2007/05/16/AR2007051601967_pf.html).