Missouri Gov. Jay Nixon said he hasn't yet learned of the jury's decision, expected to be announced at 9 p.m. ET, but he appealed for calm Monday night in a televised address.
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"Our shared hope and expectation is that regardless of the decision, people on all sides show tolerance, mutual respect and restraint," Nixon said.
"Together we are all focused on making sure the necessary resources are on hand to protect lives, protect property and protect free speech."
Nixon said local and state police have deployed their "best, most experienced" officers onto the streets in anticipation of possible turbulence, and repeated his controversial announcement from last week that National Guard personnel will be providing security at facilities like firehouses and electrical substations.
The St. Louis county prosecutor's office said Monday that the 12-person grand jury has made up its mind and that an announcement from officials would come Monday night.
The decision could touch off demonstrations similar to those seen after Wilson shot Brown dead on Aug. 9 during an attempt to arrest him. Those protests saw riot police deploy tear gas on demonstrators in footage seen around the United States.
"The world will be watching us," St. Louis Mayor Francis Slay said Monday evening.
"They are going to watch how we handle our disagreements in the coming days and how we make needed change in the coming months and years. St. Louis finds itself with an opportunity to show the nation the ways in which a community can be more fair, and more just for everyone. We must seize this opportunity together."
Federal charges not expected
Under Missouri law, the 12-person grand jury only needs nine members to find there is "probable cause" to lay a charge for it to be approved.
The panel could opt for any of a range of possible charges, from murder down to involuntary manslaughter, or could decide not to lay any charges.
The grand jury was convened in mid-August and has heard testimony and evidence since then.
The FBI is conducting a separate federal investigation into whether Wilson breached any of Brown's civil rights. That could result in additional counts, though the Washington Post, citing sources, said investigators have more or less decided federal charges are not warranted.
In his most recent statement on the matter earlier this month, departing U.S. Attorney General Eric Holder said there is no timeline for the FBI to announce the results of its probe.
Protesting on Sunday night in St. Louis, Reggie Cunningham said he doubted Wilson will be indicted and it seemed authorities were delaying an announcement "to spin this in the most positive way possible."
"The more that they drag this out, the angrier people are going to be," said Cunningham, 30, of St. Louis.
Shot multiple times
Wilson shot Brown at least six times early on a Saturday afternoon on a quiet street in Ferguson.
The incident was touched off when Wilson, driving a squad car down a street, rolled down his window to command Brown and a friend, who were walking in the middle of the road, to move onto the sidewalk.
A scuffle broke out between the office and the teen, with Brown eventually fleeing and Wilson chasing after him, gun drawn.
Details around the shooting are contested, with some witnesses and Brown's supporters saying he had his arms in the air to surrender when Wilson started firing.
Wilson, 28, reportedly told the grand jury that he feared for his life as Brown, who was 6-foot-4 and nearly 300 pounds, came at him.
It's not uncommon for deliberations to take a while in complex cases when, such as in the Brown shooting, self-defence is alleged or there are two widely conflicting versions of events, said Cole County prosecutor Mark Richardson, who is not involved in the Ferguson case.
"In the course of their deliberations, if one grand juror convinces the others that 'Look, we need to hear from an additional witness,' and they all agree, the prosecutor's got a duty to bring that witness in," Richardson said.