A grand jury has decided not to indict Ferguson police officer Darren Wilson in the death of Michael Brown, the unarmed, black 18-year-old whose fatal shooting sparked weeks of sometimes-violent protests.

St. Louis County Prosecuting Attorney Bob McCulloch announced the decision Monday evening. A grand jury of nine whites and three blacks had been meeting weekly since Aug. 20 to consider evidence. The panel met for 70 hours and heard from 60 witnesses.

McCulloch stressed that the grand jurors were "the only people who heard every witness ... and every piece of evidence." He said many witness presented conflicting statements that ultimately were inconsistent with the physical evidence.

"These grand jurors poured their hearts and soul into this process," he said.

As McCulloch was reading his statement, a crowd gathered around a car from which it was being broadcast on a stereo. When the decision was announced, Michael Brown's mother, Lesley McSpadden, who was sitting atop the car, burst into tears and began screaming before being whisked away by supporters.

The crowd erupted in anger, converging on the barricade where police in riot gear were standing. They pushed down the barricade and began pelting police with items, including a bullhorn. Police stood their ground.

At least nine votes would have been required to indict Wilson. The panel met in secret, a standard practice for such proceedings.

The US Justice Department is conducting a separate investigation into possible civil rights violations that could result in federal charges. The department also has launched a broad probe into the Ferguson Police Department, looking for patterns of discrimination.

The Aug 9 shooting inflamed tensions in the predominantly black St. Louis suburb that is patrolled by an overwhelmingly white police force.

As Brown's body lay for hours in the center of a residential street, an angry crowd of onlookers gathered. Rioting and looting occurred the following night, and police responded with armored vehicles and tear gas.

Protests continued for weeks - often peacefully, but sometimes turning violent, with demonstrators throwing rocks and Molotov cocktails and police firing smoke canisters, tear gas and rubber bullets. Missouri Gov. Jay Nixon to briefly summon the National Guard.

Hours before the announcement, the governor urged people to remain peaceful as he appeared at a news conference with the state's public safety director and the leaders of St. Louis city and county.

"Our shared hope and expectation is that regardless of the decision, people on all sides show tolerance, mutual respect and restraint," Nixon said.