A black teenager shot and killed by white St Louis police officers last week died from a single gunshot that entered his back and struck his heart, a medical examiner said on Friday, which appears to contradict the police account of the shooting.

News of the preliminary results of an autopsy escalated tensions that had flared after Wednesday’s killing of Mansur Ball-Bey, as protesters and family of the slain 18-year-old questioned police accounts that he pointed a gun at officers. In angry clashes on Wednesday night, officers in riot gear fired tear gas and more protests followed on Thursday night.

Fresh protests are planned for Friday night in the area of the shooting, according to social media posts by activists.

St Louis Police Chief Sam Dotson said additional officers would be available for the weekend in anticipation of further protests.

Late on Friday, St Louis’ elected prosecutor, Circuit Attorney Jennifer M. Joyce, said her office would begin investigating now, rather than waiting until police turn over the results of their own investigation.

She assured St Louisans at a news conference that her investigation would be independent from the one St Louis police are conducting.

“I don’t work for the police chief,” said Joyce. “I don’t work for the mayor. I am elected separately. When police have committed a crime, I hold them accountable.”

Less than two weeks ago, the St Louis area was flooded with protesters from across the country marking the anniversary of the August 9, 2014, killing of unarmed black teenager Michael Brown by a white police officer in nearby Ferguson, Missouri.