The Missouri National Guard was expected to be deployed Tuesday in and around Ferguson, Missouri, as a grand jury decision on whether to indict Darren Wilson, a white police officer, in the death of unarmed black teenager Mike Brown could be handed down at any time. Authorities are concerned that the violence that plagued early protests after Brown’s shooting in August will rear its head again should Wilson not be indicted.

Missouri Gov. Jay Nixon declared a state of emergency Monday, which allowed for the National Guard to be activated. He said local law enforcement, not the National Guard troops, would have the leading role in dispersing violent protesters, should the demonstrations come to that point.

"The National Guard is well suited to provide security at command posts, fire stations and other locations, freeing up law enforcement officers to focus on community policing and allowing citizens to exercise their constitutional rights,” the governor said, according to Reuters.

Nixon’s decision to activate the National Guard has angered leaders of the Ferguson protests who say the preemptive order is government overreach and will only heighten tensions between protesters and police. The move was also decried as a signal that authorities presumed protests would turn violent.

“The National Guard is called in when policing has failed. Military presence in my city will mark a historic failure on the part of [government],” tweeted Antonio French, a St. Louis alderman who has been a fixture at the protests. “This started on August 9th with government overreaction to black youth and it continues. This is not a war. There is not military solution.”

The FBI said it was “likely” that extremist protesters may attack authorities in the event that Wilson is not indicted, according to a bulletin obtained Monday by ABC News.

“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 bulletin said. “This also poses a threat to those civilians engaged in lawful or otherwise constitutionally protected activities.”

The National Guard is part of the U.S. armed forces reserve and serves as the first line of defense within the nation. Since its founding, the branch has been summoned during some of the most divisive moments in U.S. history, with its presence often causing tensions, especially among people of color.