A day of peaceful protests in Boston spiraled out of control within minutes on Sunday evening, triggering violence, vandalism and looting.
So how did it happen?
Todd McGhee is a retired Massachusetts state trooper who now provides security consulting and law enforcement training. He believes anarchists are trying to capitalize on the chaos and steal the attention away from the protesters' intended message.
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"The bad elements are going to find their way in and will absolutely infiltrate the good people there to protest," McGhee said. "You have to be ready for it."
McGhee was involved in the preparations and tactical training prior to the Democratic National Convention in 2004, which provided a background on the tactics used to disrupt protests.
He said antagonists know how to escalate an emotionally-charged gathering quickly. From vandalized police vehicles to rampant looting, McGhee said the same tactics are marring the message of peaceful protests across the country.
"They are using plays out of the same playbook," he said. "It preys on the emotions of the crowd. And at that point, all the attention shifts."
So how do you prevent it?
McGhee said a key is having intelligence on the ground that can identify agitators with ulterior motives and remove them from the crowd. That means having enough law enforcement resources at the scene and having an open line of communication with peaceful protesters about identifying anyone who is trying to instigate violence.
"You have specially-trained officers that know how to go into a crowd, surgically remove those antagonists, handcuff them, get them out of the way, and allow the peaceful protest to continue," McGhee said.