FERGUSON, Mo. (AP) — A grand jury has reached a decision about whether to indict a Ferguson police officer in the shooting death of Michael Brown, a spokesman for St. Louis County's top prosecutor said Monday.

In a brief email to reporters, spokesman Ed Magee said the decision would be announced at 8 p.m. at the downtown courthouse in the St. Louis County seat of Clayton. He offered no other details.

As the nation awaited the announcement, authorities quickly stepped up security around the courthouse. 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.

Several school districts canceled classes for Monday and Tuesday, extending the Thanksgiving break. More than 15 districts cancelled Monday evening activities. Washington University closed a satellite campus in Clayton.

Missouri Gov. Jay Nixon called a news conference to urge everyone involved to show respect and restraint in advance of the announcement. The governor said he did not know what the grand jury had decided.

The grand jury has been considering charges against Darren Wilson, the white suburban St. Louis officer who fatally shot the black 18-year-old after a confrontation in August.

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 traveled to St. Louis from the Capitol on Monday in advance of the grand jury announcement. He was to speak with St. Louis-area clergy on a conference call and then hold a news conference at the University of Missouri-St. Louis.

Pastors were planning a rally and prayer service later Monday at the West Side Missionary Baptist Church in St. Louis.

"There's a lot of hurt, a lot of brokenness. There's anger and frustration on every side," said the Rev. Ronald Bobo Sr., the church's pastor. "We need the hand of God to lead us and guide us."

The 12-person grand jury has been meeting in secret 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.

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Associated Press writer Alan Scher Zagier in Clayton contributed to this report. Follow David A. Lieb at: https://twitter.com/DavidALieb .