Some critics dispute that Oswald shot Tippit, arguing that the physical evidence and witness testimony do not support that conclusion. New Orleans district attorney Jim Garrison, who investigated the assassination of John F. Kennedy and brought evidence in his 1969 trial of businessman Clay Shaw, contended in his book "On the Trail of the Assassins" that the witness testimony and handling of evidence in the Tippit murder was flawed and that it was doubtful that Oswald was the killer or even at the scene of the crime. According to Garrison, numerous witnesses that were not interviewed by the Warren Commission reported seeing two men fleeing the scene of Tippit's murder. He also claimed that Helen Markham, the Warren Commission's star witness, expressed uncertainty as to her identification of Oswald in the police lineup. Garrison also claimed that bullets recovered from Tippit's body were from two different manufacturers, and the gun found on Oswald at his arrest did not match the cartridges found at the scene. Garrison accused the Dallas Police Department of mishandling the evidence and of possibly firing Oswald's revolver to produce bullet cartridges for the FBI to link to his gun.