By Carey Gillam

(Reuters) - A black teenager shot and killed by white St. Louis police officers this 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 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. He said he met with Gov. Jay Nixon and the Missouri Highway Patrol Friday to arrange for state assistance if needed.

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

Brown's death was one of a series of police killings of unarmed black men and teens across the United States that sparked a newly energized civil rights movement under the banner "Black Lives Matter".

Autopsy results show a bullet struck Ball-Bey in the upper right of his back, hitting his heart and an artery next to it, said St. Louis Chief Medical Examiner Michael Graham.

The autopsy findings appear to contradict the version of the shooting given by police, who said two officers shot at Ball-Bey when he pointed a gun at them as he fled a home where police were serving a search warrant. Police said Ball-Bey dropped his weapon and continued running after he was shot.

The position and track of the bullet, which did not exit Ball-Bey's body, show that he was not turned toward officers when he was shot, Graham said. The shot would have killed him nearly instantly, making it difficult if not impossible for him to keep running, though if he was running there would have been some forward momentum, Graham said.

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Graham said it was impossible to tell from the autopsy whether Ball-Bey was slightly turned, or was twisting his torso toward officers when he was shot.

"There are so many variables," said Graham. "But he certainly wasn't facing, his chest wasn't facing the officers."

The results of the autopsy are preliminary and evidence was still being gathered, Dotson said, but he said one witness had corroborated officer accounts that Ball-Bey had a gun.

"The complete truth takes time to put together," he told a press conference. "We must let the physical evidence lead us to our conclusions."

Police said they had recovered a gun, which they determined was stolen, though they do not know if Ball-Bey's finger prints were on it, Dotson said.

Jermaine Wooten, an attorney representing Ball-Bey's family, told CNN Friday no witnesses had seen the teenager with a gun.

Wooten said Ball-Bey did not live in the community and was visiting relatives but not at the house where police were serving the warrant, he said.

"He never had a gun. He did not point back toward the officers," Wooten told CNN. He said Ball-Bey could not have run more than a few feet after being shot, which contradicts police statements.

A report containing evidence gathered in Ball-Bey's shooting will be turned over to the city attorney and the U.S. Attorney in St. Louis for review, police officials said.

Antonio French, a St. Louis Alderman and prominent voice in the black community, called for the circuit attorney of St. Louis to conduct a simultaneous investigation of the shooting.





(Editing by James Dalgleish)