A federal grand jury chose not to indict Ferguson, Missouri police officer Darren Wilson in the August shooting death of Michael Brown.
"The duty of the grand jury is to separate fact from fiction," said St. Louis Country prosecutor Robert P. McCulloch at a press conference. "They determined that no probable cause exists to file any charge against officer Wilson."
McCulloch said physical evidence contradicted some eyewitness testimony, which he said was not always consistent.
The 12 jurors heard more than 70 hours of testimony from about 60 witnesses, he said.
The result provides some legal clarity after months of tension.
"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 a Monday night press conference.
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.