JEFFERSON CITY, Mo. >> Missouri Gov. Jay Nixon declared a state of emergency Monday and activated the National Guard in advance of a grand jury decision about whether a white police officer will be charged in the fatal shooting of a black 18-year-old in the St. Louis suburb of Ferguson.

Nixon said the National Guard would assist state and local police in case the grand jury’s decision leads to a resurgence of the civil unrest that occurred in the days immediately after the Aug. 9 shooting of Michael Brown by Ferguson Police Officer Darren Wilson.

"All people in the St. Louis region deserve to feel safe in their communities and to make their voices heard without fear of violence or intimidation," Nixon said in a written statement.

There is no specific date for a grand jury decision to be revealed, and Nixon gave no indication that an announcement is imminent. But St. Louis County Prosecutor Bob McCulloch has said that he expects the grand jury to reach a decision in mid-to-late November.

The U.S. Justice Department, which is conducting a separate investigation, has not said when its work will be completed.

Brown, who was unarmed, was shot by Wilson after some sort of confrontation occurred when the officer spotted Brown and a friend and told them to stop walking in the middle of a street.

Brown’s shooting stirred long-simmering racial tensions in the St. Louis suburb, where two-thirds of the residents are black but the police force is almost entirely white. Rioting and looting a day after the shooting led police to respond to subsequent protests with a heavily armored presence that was widely criticized for continuing to escalate tensions. At times, protesters lobbed rocks and Molotov cocktails at police, who fired tear gas, smoke canisters and rubber bullets in an attempt to disperse crowds.

Nixon also declared a state of emergency in August and put the Missouri State Highway Patrol in charge of a unified local police command. Eventually, Nixon activated the National Guard to provide security around the command center.

This time, Nixon said the St. Louis County Police Department would be in charge of a unified police command in Ferguson. The St. Louis city police and Missouri State Highway Patrol will help.

The governor did not indicate how many National Guard troops would be mobilized, instead leaving it to the state adjutant general to determine. Nixon said the National Guard would be available to carry out any requests made through the Highway Patrol to "protect life and property" and support local authorities. If the Guard is able to provide security at police and fire stations, then more police officers may be freed up to patrol the community, Nixon said.