The "second-gunman theories" aren't new. Ever since Robert Kennedy's assassination nearly 40 years ago on June 5, 1968, conspiracy theorists have argued that at least one more shooter had to be involved.
But the two forensic experts behind the recent claims say they've used new technology to analyze recordings of the assassination. And it leads them to the conclusion that another person fired the fatal shot.
Robert Kennedy's assassination is one of those moments tragically etched in American history.
Arrested and convicted of the killing, Sirhan Sirhan was said by prosecutors to be the sole gunman. But two forensic investigators say they have new evidence showing that someone else fired the bullet that actually killed Robert Kennedy.
"It can be established conclusively that Sirhan did not shoot Senator Kennedy, and in fact not only did he not do it, he could not do it," said forensic scientist Robert Joling.
Dr. Robert Joling and Philip Van Praag have studied film of the scene at the Ambassador Hotel on that June 1968 day.
But the two say a relatively obscure audio recording leads them to the conclusion that 13 shots were fired. Sirhan's gun could only hold eight bullets.
Joling and Van Praag are not alone: Other forensic specialist agree. Like the ones in a June 2007 Discovery Channel special.
And like other conspiracy theorists, the two argue that the shot that killed Kennedy had to come from the rear. Sirhan was in front of Kennedy.
The two document their argument for a second shooter in their new book "An Open and Shut Case."
"What we'd like to see is a new investigation, based on the new facts that we've come up with and to bring the authorities in," said forensic scientist Philip Van Praag.
But authorities aren't moving very quickly to dig out the RFK assassination file.
L.A. County District Attorney Steve Cooley is skeptical about the new claims. But he says his office will look into the second shooter theory.
"We'll read the book and if there's anything there that we with all our information think is legitimate, we'll pursue it," said Cooley. "But right now there's nothing to indicate that."
And the two scientists will have an even harder time convincing L.A. Police Chief William Bratton to re-open the investigation.
"That incident happened many, many years before I got here, and I have no intention of reopening that case," said Bratton.
Many forensic experts believe the "second gunman" theories are flat wrong.
They say the sounds some believe are gunshots are actually balloons popping, or the noise made by microphones banging into things during the chaos surrounding the shooting.
