A grand jury reached a decision Monday in the case against the Ferguson police officer who shot and killed an unarmed black teenager in August, touching off nationwide protests and cries of police brutality.

The Office of the Prosecuting Attorney in St. Louis County, Mo., said the decision would be revealed at 9 p.m. ET, though gave no indication whether Darren Wilson would be charged in the killing of 18-year-old Michael Brown.

As the nation awaited the announcement, authorities quickly stepped up security at the downtown courthouse in the St. Louis County seat of Clayton. Barricades were erected around the building, and more than 20 Missouri state troopers were seen silently assembling with rifles, 3-foot batons, riot shields and other equipment. Some nearby businesses boarded up their windows, just as many shops have already done near the site of Brown’s death in Ferguson.

Speaking at the University of Missouri-St. Louis Monday evening, Missouri Gov. Jay Nixon urged peace and said authorities were prepared to deal with possible protests.

“While none of us knows what that will be, our shared hope and expectation is that regardless of the decision, people on all sides show tolerance, mutual respect, and restraint,” he said. “Together we are all focused that the necessary resources are on hand to protect lives, protect property and protect free speech.”

The 12-person grand jury — which is comprised of nine whites and three blacks — has been meeting in secret once a week for months, hearing evidence from a wide variety of witnesses as it decides whether Wilson’s should face charges that could range from involuntary manslaughter to murder. The grand jurors could decide not to charge Wilson at all.

At the lower end of the possible charges is second-degree involuntary manslaughter, which is defined as acting with criminal negligence to cause a death. It is punishable by up to four years in prison. The most serious charge, first-degree murder, can be used only when someone knowingly causes a death after deliberation and is punishable by either life in prison or lethal injection.

Wilson, 28, shot and killed Brown on a Ferguson, Mo. street following a scuffle on Aug. 9 as the teenager and a friend walked back from a convenience store. Brown’s body lay in the street for four hours in the summer heat, and neighbors later lashed out at authorities, saying they mistreated the body.

Witnesses later said that Brown had his hands raised and was trying to surrender when Wilson approached with his gun and fired repeatedly. Several media organizations, citing sources they didn’t identify, have reported Wilson told grand jurors Brown was coming at him aggressively.

The shooting triggered riots and looting in and around the Ferguson area, and police responded to protesters 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.

At times, the debate surrounding the shooting has focused as much on authorities’ response — which also featured officers equipped with military style gear, including armored vehicles, body armor and assault rifles — as the shooting itself.