The FBI is warning law enforcement agencies nationwide that a grand jury’s decision on whether a Missouri police officer will face charges for killing Michael Brown, an unarmed 18-year-old, “will likely” lead to attacks on police officers and key infrastructure.

Violence could erupt following the decision whether or not to indict Ferguson Police Officer Darren Wilson, who is white, in the Aug. 9 shooting death of Brown, a black man, and so-called “hacktivists” could also use the opportunity to launch cyber-attacks against authorities and institutions.

“The announcement of the grand jury’s decision … will likely be exploited by some individuals to justify threats and attacks against law enforcement and critical infrastructure,” the FBI said in a bulletin issued Friday. “This also poses a threat to those civilians engaged in lawful or otherwise constitutionally protected activities.”

There’s no indication an announcement by the grand jury is imminent. The St. Louis County prosecutor has said that he expects the grand jury to reach a decision in mid-to-late November.

The FBI bulletin also stressed the “importance of remaining aware of the protections afforded to the all U.S. persons exercising their First Amendment rights of freedom of speech and freedom of assembly.”

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FBI officials also cited specific tactics that could be utilized by extreme protesters, including violence against state or federal authorities.

“The FBI assesses those infiltrating and exploiting otherwise legitimate public demonstrations with the intent to incite and engage in violence could be armed with bladed weapons or firearms, equipped with tactical gear/gas masks, or bulletproof vests to mitigate law enforcement measures,” the bulletin continued.

Missouri Gov. Jay Nixon declared a state of emergency and activated the National Guard Monday in advance of the grand jury decision. Nixon said the National Guard would assist state and local police as needed in the event of civil unrest.

The potential unrest over Brown's death has caused some organizations to rethink holding their conventions in the St. Louis region. The St. Louis Post-Dispatch reports the Church of God in Christ's annual convention has contributed millions of dollars to the St. Louis economy. The church's presiding bishop sent a letter to Gov. Nixon last week over concerns about Brown's fatal shooting by Wilson. The letter said the church might consider relocating its annual conference.

An official with the St. Louis Convention and Visitors Commission says they've had several customers express their concern over unrest in the St. Louis region. But he says the pace of bookings has been good.

Meanwhile, police officials in Ferguson and other cities across America braced Monday for possible violence in the wake of the decision, one day after hundreds of people took to the streets of St. Louis briefly to block a major intersection to protest the death of Brown.

Dozens of protesters could be seen lying down in the street outside of a movie theater hosting a film festival, pretending to be shot in an action intended to evoke the memory of Brown, according to Reuters.

"This is a mature movement. It is a different movement than it was in August. Then it just had anger, justifiable anger," DeRay McKesson, a 29-year-old protest leader, told Reuters.

Two protests Sunday, which marked the 100th day of demonstrations, were peaceful.

Residents and officials in the region fear another wave of rioting if the grand jury decides not to indict Wilson.

"We are bracing for that possibility. That is what many people are expecting. The entire community is going to be upset," said Jose Chavez, 46, a leader of the Latinos en Axion community group.

For some cities, a decision in the racially-charged case will, inevitably, re-ignite long-simmering debates over local police relations within minority communities.

"It's definitely on our radar," said Lt. Michael McCarthy, police spokesman in Boston, where police leaders met privately Wednesday to discuss preparations. "Common sense tells you the timeline is getting close. We're just trying to prepare in case something does step off, so we are ready to go with it."

In Los Angeles, rocked by riots in 1992 after the acquittal of police officers in the beating of Rodney King, police officials say they've been in touch with their counterparts in Missouri.

"Naturally, we always pay attention," said Commander Andrew Smith, a police spokesman. "We saw what happened when there were protests over there and how oftentimes protests spill from one part of the country to another."

In Las Vegas, police joined pastors and other community leaders this week to call for restraint at a rally tentatively planned northwest of the casino strip when a decision comes.

In Boston, a group called Black Lives Matter, which has chapters in major cities, is organizing a rally in front of the police district office in the Roxbury neighborhood the day after the grand jury's decision.

In Albuquerque, N.M., police are expecting demonstrations after dealing with a string of angry protests following a March police shooting of a homeless camper and more than 40 police shootings since 2010.

Philadelphia police spokesman Lt. John Stanford said he anticipated his city will see demonstrations, regardless of what the grand jury returns.

But big-city police departments stressed they're well-equipped to handle crowds. Many saw large but mostly peaceful demonstrations following the 2013 not-guilty verdict in the case of Florida teen Trayvon Martin, who was shot and killed by neighborhood watch coordinator George Zimmerman. In New York, hundreds of protesters marched from Union Square north to Times Square, where a sit-in caused gridlock.

The New York Police Department, the largest in the nation, is "trained to move swiftly and handle events as they come up," spokesman Stephen Davis said.

In Boston, McCarthy said the city's 2,200 sworn police officers have dealt with the range of public actions, from sports fans spontaneously streaming into the streets following championship victories to protest movements like Occupy.

"The good thing is that our relationships here with the community are much better than they are around the world," he said. "People look to us as a model. Boston is not Ferguson."

The Associated Press contributed to this report.