The unarmed black teenager killed by a police officer in Ferguson, Missouri, on 9 August was shot at least six times, including twice inthe head, a medical examiner has said.
Dr Michael Baden was hired to perform a second, independent autopsy by the family of Michael Brown, 18.
The family have called for the arrest of the officer who killed Mr Brown.
Mr Brown's death has sparked days of unrest and clashes between protesters and police in the St Louis suburb.
The officer who shot Mr Brown, Darren Wilson, has been suspended with pay since the shooting, and Mr Brown's family have called for his arrest and prosecution.
'We don't know' As the unrest, marked by a crackdown by riot police wielding tear gas and rubber bullets, has continued, on Sunday Missouri Governor Jay Nixon ordered the National Guard to the town to "help restore peace and order".
On Monday, lawyer Ben Crump said Mr Brown's family had asked Dr Baden to perform an autopsy as they "did not want to be left having to rely on the autopsy done by the St Louis law enforcement... the same individuals they feel are responsible for executing their son in broad daylight".
Shawn Parcells, a forensic pathologist who assisted Dr Baden, said a wound to Mr Brown's right arm may have been sustained as he had his hands up, "but we don't know".
He said the wound was consistent either with having his back to the officer or facing the officer with his hands above his head or in a defensive position. Witnesses have said Mr Brown was shot as he held his hands up in a position of surrender.
Dr Baden, a veteran of the New York City medical examiner's office and nationally prominent forensic pathologist, said his preliminary findings could answer the family's basic questions, including how many times he was shot and if he suffered.
"We can answer those questions on day one on the basis of the injuries," Dr Baden told reporters. "Telling that to a family can be very helpful at a trying time."
"The family has a right to know how their loved one died. This calms family or community concerns."