Bell took office in January 2019, and said he chose to reexamine the case after requests from the Brown family and community.
"There are so many points at which Darren Wilson could have handled the situation differently, and if he had, Michael Brown might still be alive", he said.
He said the reinvestigation into the actions of Wilson, who is white, for the death of Brown, who was Black - which sparked unrest in Ferguson long before America had learned the name of George Floyd - was necessary.
"The question for this office was a simple one: Could we prove beyond a reasonable doubt that when Darren Wilson shot Michael Brown he committed murder or manslaughter under Missouri law?"
However, Bell said, "Our investigation does not exonerate Darren Wilson".
Bell's office spent five months reviewing forensic reports, witness statements, and other pieces of evidence, and because there is no statute of limitations on filing murder charges and Wilson was never charged and tried, double jeopardy was not an issue, The Associated Press reports. The jury ultimately declined to indict Wilson. Bell was noncommittal until Thursday's announcement.
Wilson shot Brown, 18, on August 9, 2014, after the officer pursued the teen and his friend on suspicion of stealing from a market.
In his campaign to unseat McCulloch, Bell focused on larger criminal justice issues, not on McCulloch's handling of the Wilson investigation. Tory Russell, 36, of St. Louis screamed at the prosecuting attorney.
Multiple autopsies showed Brown was shot six times all from the front.
"It's over", he said, as police escorted him from the briefing room.
Brown's body remained in the street for four hours, angering his family and nearby residents. The next year, the U.S. Department of Justice concluded that Wilson did not violate Brown's civil rights and that no federal charges would be filed.
Bell's predecessor as prosecutor had also decided no charges were possible.
Bell, who, like McCulloch, is the son of a police officer, formed a special unit to look into officer-involved shootings like the one in Ferguson, as well as cases of potential wrongful convictions. "After an independent and in-depth review of the evidence, we can not prove that he did".
Brown's mother, Lesley McSpadden, asked Republican Gov. Mike Parson to reopen the investigation of Wilson in 2018, but Parson's office said it had no legal authority to appoint a special prosecutor.