Ferguson police Officer Darren Wilson. (Getty/Facebook)

CLAYTON, Mo. — A St. Louis County grand jury has reached a decision on whether to charge Ferguson Police Officer Darren Wilson in the shooting death of Michael Brown Jr.

A spokesman for the St. Louis prosecutor's office alerted media of the decision in a brief email statement and said it would be announced at 8 p.m. at the downtown courthouse in Clayton.

Missouri Gov. Jay Nixon delivered remarks ahead of the decision and made a plea for a peaceful response, no matter what the outcome of the grand jury.

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

Meanwhile, officials in Clayton stepped up security measures erecting barricades around the courthouse to prepare for possible protests. A Yahoo News reporter spotted members of the National Guard arriving in military vehicles. Several Ferguson-area schools have announced they will be closed Tuesday.

“The men and women of the National Guard also will be in the area to provide security at critical facilities like firehouses, police stations and utility substations, and offer logistical and transportation support as needed. This will free up law enforcement officers to do their jobs effectively," Nixon said.

Earlier Monday, a city official confirmed to Yahoo News that as of 3 p.m. CT, Wilson was still employed by the Ferguson police department and on paid leave. Several media outlets reported last week that Wilson was preparing to resign.

Anxiety over the decision has kept the St. Louis region on edge.

The grand jury convened Aug. 20 to hear evidence and testimony. The 12-member group was asked to decide if there was probable cause to charge Wilson with a crime and what that charge should be. Options range from second-degree involuntary manslaughter to second-degree murder. Wilson, 28, could face life in prison if convicted of the most serious charge.

Photos surround Michael Brown's casket before the start of his funeral at Friendly Temple Missionary Baptist Church on Aug. 25. (AFP/Richard Perry)

But the panel may find that probable cause does not exist and dismiss the state’s case, without charging Wilson.

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Many, including Wilson through a police union spokesman, have recently expressed a belief that the grand jury will not hand down an indictment.

On Thursday, Jeff Roorda, representative with the St. Louis Police Officers' Association, described Wilson as being under a lot of pressure and stress but confident in the outcome of the grand jury investigation.

“It's fair to say that neither he nor his defense team expect an indictment,” Roorda told the AP, offering his impression of the situation based his meeting with Wilson.

The Aug. 9 shooting led to weeks of protests and saw citizens and heavily armed police clash. Stores in Ferguson were looted and property was destroyed. Hundreds were arrested.

[Related: Timeline of events after the death of Michael Brown]

Wilson has reportedly told investigators that he feared for his life and felt justified in fatally shooting the 18-year-old. He said Brown assaulted him inside his patrol SUV and tried to take his gun. The teen’s family and some witnesses say that Wilson was the aggressor and ultimately killed Brown as he raised his hands in surrender.

Activists responded to decision announcement by reminding their followers that marches will be held in Ferguson, Clayton and St. Louis' historic Shaw neighborhood.

not the time to panic y'all. we've been planning. we are prepared either way. either way. — ShordeeDooWhop (@Nettaaaaaaaa) November 24, 2014

Officials have been planning for months for a possible backlash after the grand jury announces its decision. Missouri Gov. Jay Nixon activated the state’s National Guard and declared a state of emergency. The governor’s order directs police to “to protect civil rights and ensure public safety in the City of Ferguson and the St. Louis region.”

Michael Brown, Sr., the teen's father, issued a plea for peace late Thursday.

“No matter what the grand jury decides, I do not want my son’s death to be in vain — I want it to lead to positive change,” a very somber Brown says in a public service announcement. “I thank you for lifting your voices to end racial profiling and police intimidation, but hurting others or destroying property is not the answer.”

[Related: No promise Ferguson grand jury evidence will be made public, court says]

Activists from around the country have been arriving in the area for weeks to help plan demonstrations. Some groups aim to keep the protests peaceful and have asked police to follow proposed “rules of engagement.” Other organizations are issuing protective equipment and have set up “safe spaces” for protesters.

Brown graduated from high school in May and was planning to start trade school on Aug. 11, two days after he was shot. He was buried Aug. 25 in St. Louis County.

Wilson, an officer in Ferguson for three years, has been on paid leave and has not been seen in public since the shooting. Ferguson Police Chief Tom Jackson recently told Yahoo News that Wilson would “immediately” return to active duty if he was not indicted but was likely to be terminated if he was charged with a felony.

(This is a developing story. Please check back for updates.)

Jason Sickles is a reporter for Yahoo. Follow him on Twitter (@jasonsickles).