When demonstrators started throwing bottles and planks of wood at police who were lining Cermak and Michigan, it wasn't a surprise, based on our review of sometimes dramatic communications between special dispatchers in a command center a few miles away and field commanders on the scene.
Chicago police radio: "We have intel by the stage, people are preparing to throw things at the police ? the Black Bloc have a sheet of paper and they are gathering and making a plan, FYI."
The Black Bloc is a group of about 200 men and women dressed in black most covering their faces known to be violent at previous protests. They had left the march area but then returned.
Chicago police radio: "The black bloc are gearing up, they have eye and ear protection...we're all geared up and waiting your command."
"It's the best listening I've heard," said Dave Weaver, Radioman911.com
Weaver's website streams local public safety communications. Radioman911.com hit its highest number of viewers ever, when protestors collided with police just after the march finished.
"When you hear what they had to deal with and how intense it was, you understand that their job is so critical to our own safety and we are lucky to be in this city with that type of expertise," said Weaver.
As tensions escalated, that expertise helped police at the front lines know what was going on around them.
Chicago police radio: "The Black Bloc, if you can't see them, they are in the middle holding those flags up. Do not lose them, stay with them, you are doing perfect ... use caution, be alert, all eyes on the response team get to the vehicles and get your helmets, watch out they're giving hand signals. Reinforce that fence line."
With that, from our high camera, it was evident that the Black Bloc was moving as a wedge toward the police line. By speeding up our video, you can see the group's intentional push to the front lines. Once there, they took up positions facing officers and so began the clash.
Chicago police radio: "They got a lot of things to throw at the officers, rocks coming into the crowd ... NATO units, use caution, they're throwing rocks now ... Full on push, full on push ... You're on camera guys, stay with your training. You're on camera, stay with your training .... Forward march. Forward march...State Police is coming behind you. There's a lot of resources coming ... You guys are looking good, you're looking good. You're on camera. Stick with your training. Anyone that is going to batter a police officer, you grab them and you pull them through ... Officers, officers, if someone batted a police officer, you pull him through to the arresting side, you stick with your training. Stick with your training, you're on live TV."
Our cameras caught a member of the bomb and arson squad looking through trash cans near Cermak and Michigan.
"What is very significant is one of our undercovers saw what he believed to be an incendiary device in a backpack," said Supt. Garry McCarthy, Chicago Police Department. "There was a liquid in there that we're having tested and I haven't gotten results yet."
The person suspected of placing it there was taken into custody.
Chicago police radio:"Reinforce your line, like your training, reinforce your line, if a policeman gets battered you pull them through, pull them through and let the other guys take care of it. You're on TV, just stick with your training, you're looking good ... they want an ambulance to come northbound on State Street to Cermak, northbound on State Street for an injured civilian ... Injured officer 24th and State, injured officer, we'll get an ambulance rolling ... Right now the hot zone is running up from Cermak and Michigan. That's a bad area right now. Avoid that area as much as you can. We want the message to say to move west, we want the message to say that they need to move west, to disperse."
After Sunday's skirmish on the South Side, attorneys representing protesters said that they documented at least 50 instances of police brutality. On social media sites, protesters made additional claims of brutality. But the Independent Police Review Authority says only three people have come forward to file formal complaints of excessive force. An additional complaint was filed by someone who said police were "rude."