Story highlights FBI issues bulletin, urges law enforcement to be vigilant, source says

Gov. Jay Nixon says Ferguson Police will have reduced role if there is unrest

Michael Brown, a black teen, was fatally shot by Darren Wilson, a white officer

A grand jury is weighing whether to indict Wilson

Ahead of a grand jury's decision on whether to indict a police officer in the killing of Michael Brown, Missouri has both called in the National Guard and diminished the role of the Ferguson Police Department.

Missouri Gov. Jay Nixon declared a state of emergency Monday as a precaution, he said, in the event of unrest or violence.

It's unknown when the grand jury will hand down a decision on whether to indict Ferguson Police Officer Darren Wilson for his fatal shooting of Brown, a teen. Prosecutors have suggested the grand jury would be done deliberating in mid- to late November.

At the national level, the FBI last week issued a bulletin to law enforcement urging vigilance in the days before the Ferguson grand jury decision, according to a law enforcement official.

JUST WATCHED St. Louis Chief: Safety is the #1 issue Replay More Videos ... MUST WATCH St. Louis Chief: Safety is the #1 issue 02:50

JUST WATCHED Missouri Gov. activates National Guard Replay More Videos ... MUST WATCH Missouri Gov. activates National Guard 03:56

JUST WATCHED Ferguson bracing for grand jury decision Replay More Videos ... MUST WATCH Ferguson bracing for grand jury decision 03:38

The bulletin did not cite any specific intelligence to suggest there was any ongoing threat to officers. It was based on what had occurred during previous protests.

The bulletin expressed concern over possible violence relating to Ferguson protests and warning officers to be vigilant. In addition, it also mentioned that officers should take proper precautions and be vigilant over possible hacking of their personal information, the law enforcement official said.

'Possibility of expanded unrest'

Nixon said he signed the executive order because of the "possibility of expanded unrest."

Nixon said people have the right to protest peacefully but that citizens and businesses must be protected from violence and damage.

St. Louis Mayor Francis Slay said the governor's plan will bring National Guard troops to St. Louis County, where Ferguson is located, but he said their role is designed to be secondary to local law enforcement.

As Missouri prepared to deploy the troops -- officials did not say how many -- the role of the Ferguson Police Department in the case of unrest in that city will be reduced.

In the event of unrest in Ferguson, that city's police will not be at the forefront of a response, Nixon said in a teleconference with reporters. The St. Louis County Police Department will lead any response, and they will be the ones coordinating with the National Guard, if needed.

JUST WATCHED Audio provides clues to Ferguson shooting Replay More Videos ... MUST WATCH Audio provides clues to Ferguson shooting 02:16

JUST WATCHED Mayor agrees with Nixon on National Guard Replay More Videos ... MUST WATCH Mayor agrees with Nixon on National Guard 01:39

Photos: Photos: Emotions run high in Ferguson Photos: Photos: Emotions run high in Ferguson Protesters march in Ferguson, Missouri, on Thursday, August 21 2014. The St. Louis suburb saw turmoil after a white police officer, Darren Wilson, fatally shot an unarmed black teenager, Michael Brown, on August 9, 2014. Some protesters and law enforcement officers clashed in the streets, leading to injuries and arrests. Hide Caption 1 of 76 Photos: Photos: Emotions run high in Ferguson A crowd gathers in Ferguson on August 21, 2014. With the situation appearing to calm, Gov. Jay Nixon ordered the Missouri National Guard to begin withdrawing from the city. Hide Caption 2 of 76 Photos: Photos: Emotions run high in Ferguson Police escort a Wilson supporter to a police vehicle, away from crowds protesting Brown's death on Wednesday, August 20, 2014. Police have said Brown and Wilson struggled over the officer's gun; some witnesses said Brown had his hands in the air when he was shot. Hide Caption 3 of 76 Photos: Photos: Emotions run high in Ferguson Lightning streaks over protesters on August 20, 2014. Hide Caption 4 of 76 Photos: Photos: Emotions run high in Ferguson Demonstrators protest on August 20, 2014. Hide Caption 5 of 76 Photos: Photos: Emotions run high in Ferguson U.S. Attorney General Eric Holder meets with Capt. Ron Johnson of the Missouri State Highway Patrol at a Ferguson restaurant on August 20, 2014. Holder came to Missouri to talk to community leaders and review the federal civil rights investigation into Brown's shooting. Hide Caption 6 of 76 Photos: Photos: Emotions run high in Ferguson Protesters march through the streets of Ferguson on August 20, 2014. Hide Caption 7 of 76 Photos: Photos: Emotions run high in Ferguson Police point out a demonstrator who has his arms raised before moving in to arrest him Tuesday, August 19, 2014. Hide Caption 8 of 76 Photos: Photos: Emotions run high in Ferguson People watch from inside a restaurant as protesters rally August 19, 2014. Hide Caption 9 of 76 Photos: Photos: Emotions run high in Ferguson Police arrest a demonstrator on August 19, 2014. Hide Caption 10 of 76 Photos: Photos: Emotions run high in Ferguson Police charge into the media work area with weapons drawn as they try to control demonstrators on August 19, 2014. Hide Caption 11 of 76 Photos: Photos: Emotions run high in Ferguson A protester speaks to a police officer on August 19, 2014. Hide Caption 12 of 76 Photos: Photos: Emotions run high in Ferguson People on August 19 stand near a memorial where Brown was shot and killed, 2014. Hide Caption 13 of 76 Photos: Photos: Emotions run high in Ferguson Protesters march on August 19, 2014. Hide Caption 14 of 76 Photos: Photos: Emotions run high in Ferguson Police watch as protesters march August 19, 2014. Hide Caption 15 of 76 Photos: Photos: Emotions run high in Ferguson A demonstrator is arrested on August 19, 2014. Hide Caption 16 of 76 Photos: Photos: Emotions run high in Ferguson Police try to control protesters on Monday, August 18, 2014. Hide Caption 17 of 76 Photos: Photos: Emotions run high in Ferguson Police lead a man away during a protest August 18, 2014. Hide Caption 18 of 76 Photos: Photos: Emotions run high in Ferguson A demonstrator shouts during a protest on West Florissant Avenue, one of Ferguson's main streets, on August 18, 2014. Hide Caption 19 of 76 Photos: Photos: Emotions run high in Ferguson Capt. Ron Johnson of the Missouri State Highway Patrol pulls his men back from aggressive protesters on August 18, 2014. Hide Caption 20 of 76 Photos: Photos: Emotions run high in Ferguson Police arrest a demonstrator August 18, 2014, after peaceful protests gave way to angry confrontations with authorities. Hide Caption 21 of 76 Photos: Photos: Emotions run high in Ferguson Law enforcement officers stand guard during a protest on West Florissant Avenue on August 18, 2014. Hide Caption 22 of 76 Photos: Photos: Emotions run high in Ferguson A woman helps a man affected by tear gas August 18, 2014. The situation overnight deteriorated after a handful of protesters threw rocks, bottles and Molotov cocktails at police. Officers responded by firing stun grenades and tear gas canisters. Hide Caption 23 of 76 Photos: Photos: Emotions run high in Ferguson Officers stand with weapons drawn during a protest on West Florissant Avenue on August 18, 2014. Hide Caption 24 of 76 Photos: Photos: Emotions run high in Ferguson Musician Nelly, center, joins demonstrators in Ferguson on August 18, 2014. Hide Caption 25 of 76 Photos: Photos: Emotions run high in Ferguson Demonstrators receive red roses as they protest August 18, 2014. Hide Caption 26 of 76 Photos: Photos: Emotions run high in Ferguson Demonstrators march on August 18, 2014. Hide Caption 27 of 76 Photos: Photos: Emotions run high in Ferguson The Rev. Jesse Jackson shakes hands with a police officer as he visits Ferguson's demonstration area on August 18, 2014. Hide Caption 28 of 76 Photos: Photos: Emotions run high in Ferguson A protester picks up a tear gas canister on Sunday, August 17, 2014. Hide Caption 29 of 76 Photos: Photos: Emotions run high in Ferguson Police wait to advance after using tear gas to disperse protesters August 17, 2014. Hide Caption 30 of 76 Photos: Photos: Emotions run high in Ferguson Water gets poured into a woman's eyes after a tear gas attack by police on August 17, 2014. Hide Caption 31 of 76 Photos: Photos: Emotions run high in Ferguson A man runs through clouds of tear gas on August 17, 2014. Hide Caption 32 of 76 Photos: Photos: Emotions run high in Ferguson Protesters react as police fire tear gas at them August 17, 2014. Hide Caption 33 of 76 Photos: Photos: Emotions run high in Ferguson Police advance through a cloud of tear gas on August 17, 2014. Most of the crowd had dispersed after a curfew went into effect at midnight, St. Louis County authorities said. Hide Caption 34 of 76 Photos: Photos: Emotions run high in Ferguson People take cover from tear gas inside a McDonald's on August 17, 2014. Hide Caption 35 of 76 Photos: Photos: Emotions run high in Ferguson A large group of police officers advance toward protesters on August 17, 2014. Hide Caption 36 of 76 Photos: Photos: Emotions run high in Ferguson A man fights the effects of tear gas in Ferguson on August 17, 2014. Hide Caption 37 of 76 Photos: Photos: Emotions run high in Ferguson Michael Brown Sr. and Lesley McSpadden, the parents of Michael Brown, attend a rally at Greater Grace Church in Ferguson on August 17, 2014. Hide Caption 38 of 76 Photos: Photos: Emotions run high in Ferguson The Rev. Al Sharpton hugs McSpadden during the rally. Hide Caption 39 of 76 Photos: Photos: Emotions run high in Ferguson Capt. Ron Johnson of the Missouri State Highway Patrol speaks at the rally. He had been appointed by the governor to take control of security operations. Hide Caption 40 of 76 Photos: Photos: Emotions run high in Ferguson Demonstrators defy a curfew early on August 17, 2014. Hide Caption 41 of 76 Photos: Photos: Emotions run high in Ferguson Police fire tear gas at demonstrators after curfew on August 17, 2014. Hide Caption 42 of 76 Photos: Photos: Emotions run high in Ferguson A law enforcement officer aims his rifle August 17,, 2014, after tear gas was fired to disperse a crowd. Hide Caption 43 of 76 Photos: Photos: Emotions run high in Ferguson Law enforcement officers check a building on August 17, 2014. Hide Caption 44 of 76 Photos: Photos: Emotions run high in Ferguson People attend a protest after the midnight curfew on August 17, 2014. Hide Caption 45 of 76 Photos: Photos: Emotions run high in Ferguson A law enforcement officer watches as tear gas is fired to disperse a crowd on August 17, 2014. Hide Caption 46 of 76 Photos: Photos: Emotions run high in Ferguson Police stand guard before the midnight curfew on Saturday, August 16, 2014. Hide Caption 47 of 76 Photos: Photos: Emotions run high in Ferguson People loot the Ferguson Market and Liquor store on August 16, 2014. Several businesses were looted as police held their positions nearby. Hide Caption 48 of 76 Photos: Photos: Emotions run high in Ferguson Missouri State Highway Patrol officers listen to taunts from demonstrators during a protest on Friday, August 15, 2014. Hide Caption 49 of 76 Photos: Photos: Emotions run high in Ferguson Demonstrators protest with their hands up on August 15, 2014. The "hands up" gesture has become a symbol in protests as Brown, according to eyewitnesses, was trying to surrender when he was shot multiple times. Hide Caption 50 of 76 Photos: Photos: Emotions run high in Ferguson Police confront demonstrators on August 15, 2014. Hide Caption 51 of 76 Photos: Photos: Emotions run high in Ferguson A demonstrator throws a tear gas canister back at police on August 15, 2014. Hide Caption 52 of 76 Photos: Photos: Emotions run high in Ferguson Police confront demonstrators on August 15, 2014. Hide Caption 53 of 76 Photos: Photos: Emotions run high in Ferguson ri – Thousands of demonstrators march down a Ferguson street with members of the St. Louis County Police and the Missouri State Highway Patrol on Thursday, August 14, 2014. Hide Caption 54 of 76 Photos: Photos: Emotions run high in Ferguson Demonstrators hold signs as traffic moves slowly past them on August 14, 2014. Hide Caption 55 of 76 Photos: Photos: Emotions run high in Ferguson People scramble as police fire tear gas on August 17, 2014. Hide Caption 56 of 76 Photos: Photos: Emotions run high in Ferguson State Highway Patrol Capt. Ron Johnson smiles at demonstrators on August 14, 2014. Johnson was appointed to lead security as state troopers took over after days of clashes between protesters and local police. Hide Caption 57 of 76 Photos: Photos: Emotions run high in Ferguson The Rev. Traci Blackmon uses a megaphone to talk to a large group of demonstrators on August 14, 2014. Hide Caption 58 of 76 Photos: Photos: Emotions run high in Ferguson A man picks up a flaming bottle and prepares to throw it as a line of police advance in the distance on Wednesday, August 13, 2014. Hide Caption 59 of 76 Photos: Photos: Emotions run high in Ferguson Police stand in clouds of smoke as they clash with protesters on August 13, 2014. Hide Caption 60 of 76 Photos: Photos: Emotions run high in Ferguson An Al-Jazeera television crew runs for cover as police fire tear gas at its position on August 13, 2014. Video and images on social media showed police later breaking down the journalists' gear. Hide Caption 61 of 76 Photos: Photos: Emotions run high in Ferguson A protester runs from tear gas exploding around him on August 13, 2014. Hide Caption 62 of 76 Photos: Photos: Emotions run high in Ferguson Police stand guard among demonstrators on August 13, 2014 Hide Caption 63 of 76 Photos: Photos: Emotions run high in Ferguson Ferguson Police Chief Thomas Jackson fields questions during a news conference on August 13, 2014. Hide Caption 64 of 76 Photos: Photos: Emotions run high in Ferguson A small group of protesters block traffic in the street before police arrived on August 13, 2014. Hide Caption 65 of 76 Photos: Photos: Emotions run high in Ferguson A protester throws a tear gas canister back toward police on August 13, 2014. Hide Caption 66 of 76 Photos: Photos: Emotions run high in Ferguson Police detain a man on Tuesday, August 12, 2014. Hide Caption 67 of 76 Photos: Photos: Emotions run high in Ferguson People congregate at the Greater St. Marks Family Church in St. Louis along with the family of Michael Brown and the Rev. Al Sharpton on August 12, 2014. Hide Caption 68 of 76 Photos: Photos: Emotions run high in Ferguson Michael Brown Sr. stands alongside Sharpton, right, during a news conference in St. Louis on August 12, 2014. Hide Caption 69 of 76 Photos: Photos: Emotions run high in Ferguson Demonstrators protest August 12 in Ferguson, 2014. Hide Caption 70 of 76 Photos: Photos: Emotions run high in Ferguson A makeshift memorial sits in the middle of the street where Michael Brown was shot and killed. Hide Caption 71 of 76 Photos: Photos: Emotions run high in Ferguson A woman tries to calm an emotional protester during a demonstration outside the headquarters of the Ferguson Police Department on August 11, 2014. Hide Caption 72 of 76 Photos: Photos: Emotions run high in Ferguson Phaedra Parks, left, comforts Desuirea Harris, the grandmother of Michael Brown, during a news conference in Jennings, Missouri, on August 11, 2014. Hide Caption 73 of 76 Photos: Photos: Emotions run high in Ferguson Police officers arrest a man who refused to leave when police cleared streets in Ferguson on August 11, 2014. Hide Caption 74 of 76 Photos: Photos: Emotions run high in Ferguson A burned-out QuikTrip gas station smolders on August 11, 2014 after protesters looted and burned the Ferguson building the night before. Hide Caption 75 of 76 Photos: Photos: Emotions run high in Ferguson Police officers and protesters confront each other on Saturday, August 9, the same day Michael Brown was shot and killed. Hide Caption 76 of 76

The governor expects the role of Ferguson officers will be to handle routine calls not related to protests or unrest.

Brown, a black teen, was fatally shot by Wilson, a white police officer, on August 9. The grand jury is weighing whether to indict Wilson.

'That ugliness ... cannot be repeated'

"In the days immediately following Michael Brown's death, peaceful protests were marred by senseless acts of violence and destruction," the governor said last week.

"That ugliness was not representative of Missouri, and it cannot be repeated," said Nixon.

The state of emergency will expire in 30 days unless extended by another order.

Slay said the exact positions of the National Guard troops has not been decided, but he said the troops will "not be on the front lines, confronting protesters."

They will play a secondary role, he said, and will be partnered with local law enforcement anywhere they are posted.

While Slay emphasized the back-up role of the National Guard, he said he agreed with the governor's decision to declare a state of emergency.

The troops are needed for visibility and deterrence, Slay said.

There are too many unknowns about what the grand jury decision might be and how the public may react, he said.

Slay said it makes sense to be prepared in case there is violence.

The governor said the goal is to keep citizens safe while protecting people's right to protest.