The investigation did not exonerate Wilson, Bell said.
The shooting by Wilson of Brown, a black man, in August 2014 sparked a federal civil rights inquiry, protests and a national debate. Protests erupted across the country after a grand jury refused to indict Wilson.
“While this case represents one of the most significant moments in St. Louis history, the question for this office was straightforward,” Bell said. “Could we prove beyond a reasonable doubt that when Darren Wilson shot Michael Brown, he committed murder or manslaughter under Missouri law?
“After an independent and thorough review of the evidence, we cannot prove that he did,” Bell said.
“I also want to be clear that our investigation does not exonerate Darren Wilson,” Bell said, stressing the difference between proving a case during the trial and “clearing it of any wrongdoing.”
“There are so many points where Darren Wilson could have handled the situation differently, and if he had, Michael Brown could still be alive,” Bell said.
“But that’s not the issue before us. The only question is whether we can prove beyond a reasonable doubt that a crime was committed, ”Bell said. “The answer to this question is ‘no’. And I would be violating my ethical obligations if I nonetheless brought charges. ”
CNN has reached out to Brown’s family for comment.
The family filed a wrongful death lawsuit against the city and reached a settlement in 2017.
CNN’s Theresa Waldrop contributed to this report.