The prosecutor for St. Louis said Thursday she won't be charging two officers in the fatal shooting of a black 18-year-old last year, concluding that no evidence disproves claims by police that it was self-defense.

Calling Mansur Ball-Bey's August 2015 death "a tragedy in every aspect of the word," Circuit Attorney Jennifer Joyce said the officers and a witness reported that an armed Ball-Bey ran from a home during a drug and gun raid. Both officers, who are white, have said they fired at Ball-Bey at the same time after he pointed a gun at one of them, though one officer missed, according to Joyce.

Joyce, whose office investigated the shooting separately from an internal police probe, said Ball-Bey's loaded gun was found at the scene, with his palm print on the ammunition clip.

A local medical examiner concluded that Ball-Bey sustained a severed spinal cord, and a bullet pierced his heart.

"One of the biggest challenges we face in this case is that there is no independent, credible witness we can put in front of a grand jury or regular jury who contradicts police statements," Joyce said in a statement. "None of the other witnesses had a clear view at the moment when Ball-Bey was shot."

She noted that the officers declined to speak to prosecutors.

An attorney for Ball-Bey's family, Jermaine Wooten, has questioned the police account that Ball-Bey was armed. Wooten said Thursday before Joyce's announcement that he anticipated the officers would not be prosecuted, saying he has "been down this road before" with area police being cleared in fatal shootings involving blacks.

Wooten did not immediately return messages left on his cellphone after the announcement.

Ball-Bey's death came a little more than a year after a white police officer, Darren Wilson, shot and killed Michael Brown, a black, unarmed 18-year-old, in nearby Ferguson, Missouri. Brown's August 2014 shooting sparked waves of protests, including some that turned violent, and was a catalyst for the national Black Lives Matter movement and debate over police treatment of minorities.

A St. Louis County grand jury and the U.S. Department of Justice ultimately cleared Wilson, who resigned in November 2014.

Ball-Bey's death led to an outcry as well, with protests leading to arrests and damaged property.

St. Louis Police Chief Sam Dotson said Thursday he was confident Joyce made her decision after "a comprehensive review." He said his department plans to assess possible tactical lessons that could be learned.

"I have pledged transparency to the citizens of St. Louis and will continue to uphold this promise," Dotson said in a statement.

The law gives police officers latitude to use deadly force when they feel physically endangered. The Supreme Court held in a 1989 case that the appropriateness of use of force by officers "must be judged from the perspective of a reasonable officer on the scene," rather than evaluated through 20/20 hindsight.

That standard is designed to take into account that police officers frequently must make split-second decisions during fast-evolving confrontations, and should not be subject to overly harsh second-guessing. The Justice Department cited that legal threshold last year when clearing Wilson in Brown's shooting.

Last November, Joyce cleared two St. Louis police officers in the fatal shooting of 25-year-old Kajieme Powell, who in August 2014 was armed with a steak knife as he approached the officers and urged them to shoot him. The white officers fired 12 times, killing Powell, who was black, while the region already was on edge over Brown's death 10 days earlier.

But last month, Joyce charged former St. Louis police officer Jason Stockley with first-degree murder in a 2011 shooting of a 24-year-old black man. Joyce said Stockley, who is white, was on duty when police say he witnessed a drug deal involving Anthony Lamar Smith. She said after a chase with speeds exceeding 80 mph, Stockley approached the car that was being pursued and fired five times into its driver's side, striking Smith with each round.

Stockley's attorneys have said he fired in self-defense.