A grand jury chose not to indict Ferguson, Missouri police officer Darren Wilson in the August shooting death of Michael Brown.
The decision was followed by violent protests, looting and clashes with St. Louis police, who responded with tear gas, riot gear and armored vehicles.
"The duty of the grand jury is to separate fact from fiction," said St. Louis County prosecutor Robert P. McCulloch at a press conference. "They determined that no probable cause exists to file any charge against officer Wilson."
That Wilson shot Brown to death was not in question, McCulloch said. The issue before the grand jury was if lethal force was justified for the police officer. The 12 jurors heard more than 70 hours of testimony from about 60 witnesses, he said.
McCulloch said physical evidence showed Brown was charging at Wilson when he was shot. That contradicted some eyewitness testimony that Brown was surrendering. Presented with physical evidence, he said some eyewitness accounts then changed.
President Barack Obama, speaking at the White House after the decision, called on protesters to refrain from violence. He also said the Ferguson case exposed deep racial issues.
"This isn't just an issue for Ferguson, this is an issue for America," Obama said.
The grand jury result provides some legal clarity after months of tension. However, state and local officials were prepared for protests, even violence.
"Our shared hope and expectation is that regardless of that decision people on all sides show tolerance, mutual respect and restraint," said Missouri Governor Jay Nixon at Monday press conference hours before the grand jury announcement.
Nixon declared a state of emergency ahead of the decision and activated the Missouri National Guard.
Wilson is white, and Brown, 18, is black and was unarmed at the time of his death. Weeks of protests followed the shooting, and unrest included looting and rioting.
Some businesses and schools were closed in anticipation of the grand jury decision.
Demonstrators would be allowed to express themselves, said St. Louis Mayor Francis Slay, even if it means inconvenience to the city such as slowed traffic.
"But turning violent and damaging property will not be tolerated," Slay said.
The police response to the initially peaceful protests drew criticism for the use of tear gas, riot gear and armored vehicles.
A small number of protesters attacked squad cars, tossed molotov cocktails and shot at officers, who responded with tear gas, smoke canisters and rubber bullets.
More than 1,000 police officers have received additional training ahead of the decision. A combined command of state, St. Louis County and St. Louis city will provide security for expected protests. The National Guard will be made available if needed.
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Information from The Associated Press is used in this report.