Police announced the arrest of Ross shortly before a jury convicted Van Dyke of second-degree murder and 16 counts of aggravated battery with a firearm in the 2014 death of 17-year-old Laquan McDonald.
Jurors also convicted him of aggravated battery, but acquitted him of official misconduct.
Special prosecutor Joseph McMahon had told the jury that "under very special circumstances" a police officer can legally shoot a person but Van Dyke was charged with first-degree murder because "not a single shot was necessary or justified".
Jason Van Dyke, a white police officer, shot McDonald 16 times after responding to a call about a teenager breaking into vehicles in a trucking yard. "He made it up".
Chicago is bracing for protests. Eighteen police officers were charged with either murder or manslaughter in connection with an on-duty shooting in 2015, still the highest total since 2005, according to data collected by Philip Stinson, an associate professor of criminal justice at Bowling Green State University. The jury also had seven whites, three Hispanics and one Asian-American.
"If the court lets Van Dyke walk free, that would be illegitimate violence too - a threat against every black and brown youth in Chicago", they said.
Van Dyke is now being held in custody.
They were waiting for someone to arrive with a stun gun to use on the teenager when 40-year-old Van Dyke showed up.
At that point, Van Dyke shot McDonald and he fell to the ground.
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The department said in a statement that it had "a comprehensive operating plan to ensure public safety in all of our neighbourhoods while simultaneously protecting the rights of peaceful demonstrations". If Van Dyke had to serve six for each of the 16 counts and do so one sentence after another that would add up to 96 years. Jurors were told only that Walsh was testifying under "use immunity", meaning his testimony can't be used against him as long as he was truthful, but were never told about the allegations he faces.
Two alternate jurors - a man and a woman - told reporters after being dismissed by the judge that they would have found Van Dyke guilty in the shooting.
The officer had been on the force for 13 years. "At any point throughout that 20-some-minute rampage, had Laquan McDonald dropped that knife, he'd be here today". "Cases, such as Laquan McDonald, Mike Brown, Alton Sterling, [Sandra Bland] and Philando Castile are brutal illustrations on why we need a clear documentation of facts when citizen-encounters with police turn deadly".
Herbert said there was no question the case is tragic but it did not amount to murder.
Herbert argued that McDonald was to blame for what happened that night, saying "the tragedy. could have been prevented by one simple step". An extra 4,000 officers will be on the street, according to spokesman Anthony Guglielmi.
The Justice Department launched an investigation, concluding in a scathing report past year that the department violates the constitutional rights of residents.
It has placed officers on 12-hour shifts in anticipation of the trial's outcome.
Initially, Emanuel said that the U.S. Department of Justice should not investigate the CPD.