Michael Brown, the black teenager shot by police on Saturday in Ferguson, Missouri, was a suspect in a robbery just moments earlier, police have said.
But they said the robbery in a convenience store had nothing to do with the initial contact between Mr Brown and the officer who shot him.
Mr Brown's family have reacted angrily at what they see as a police attempt to "justify the execution-style murder".
The killing sparked days of angry clashes between police and protestors.
Ferguson Police Chief Thomas Jackson said the officer who shot Michael Brown did not know that he had allegedly stolen goods from a nearby store.
"The initial contact between the officer and Michael Brown was not related to the robbery," he said.
Instead, he added, the reason for the stop was that Brown "was walking down the middle of the street stopping traffic".
Mr Jackson said he had released the footage of the crime only because he had been forced to do so because of numerous freedom of information requests from journalists.
He described the robbery at the convenience store as being committed without the use of a weapon.
CCTV pictures in a police report show a well-built black man intimidating a clerk in the store.
The police report identifies the man as Mr Brown, described as being 6ft 4in (193cm) and 292 pounds (132kg).
It says he "grabbed a box of Swisher Sweet cigars" and handed them to a second suspect identified as Mr Brown's friend, Dorian Johnson.
Since the shooting on Saturday, Mr Johnson has given a number of interviews in which he has said Mr Brown held up his hands in an act of surrender before being shot by the officer.
Also on Friday Mr Jackson revealed the name of the officer who shot and killed Mr Brown - Darren Wilson - and said he was injured in a struggle with Mr Brown.
Mr Wilson, is a six-year veteran of the police force with no disciplinary record, Mr Jackson said.
Mr Brown's family accused the police of being devious in their release of information.
"There is nothing based on the facts that have been placed before us that can justify the execution-style murder of their child by this police officer as he held his hands up, which is the universal sign of surrender," said a statement.
Members of the majority black Ferguson community have also reacted angrily.
Ferguson resident Milton Jackson, 37, said: "I don't believe what the officer did was called for. Even if there was a robbery, it was unnecessary force to shoot an unarmed black man."
On Thursday night, tension seemed to ease, at least momentarily, as state police took charge of security after days of violent clashes between protesters and police.
There was no sign of the heavily armed police presence of previous nights.
US Attorney General Eric Holder said the use of military equipment and vehicles in Ferguson had sent a "conflicting message", while President Barack Obama said there was no excuse for police using "excessive force".
Mr Obama has promised a full investigation by the US Department of Justice into the teenager's death, and the FBI has launched its own inquiry.