Ferguson killing: how eyewitnesses can be blind to the truth

The sombre, grey-haired prosecutor stood before throngs of cameras on Monday night and considered the many problems that plagued the investigation into the killing of unarmed teenager Michael Brown.

There was the media and "its insatiable appetite for something, for anything to talk about". There was social media and its "nonstop rumours". And there were the dozens of witnesses.

All claimed to have seen the final moments of Brown's life, St Louis County Prosecutor Robert McCulloch said. All claimed to have witnessed Ferguson police officer Darren Wilson pump bullets into the youth. But that's where the similarities ended.

"At least one witness stated that as Officer Wilson got out of his vehicle, he shot Mr Brown multiple times as Mr Brown stood next to the vehicle," McCulloch said. "Yet another witness stated that Officer Wilson stuck his gun out of the window and fired at Mr Brown as Mr Brown was running. One witness stated there were actually two police vehicles and four officers present, but only one officer fired a weapon."

Of all the tools at an investigator's disposal - DNA analysis, forensic evidence, witness testimony - by far the least reliable are the recollections of witnesses.

Asked if he was contemplating perjury charges against any witness, McCulloch said not. "I think they truly believe that's what they saw, but they didn't."