BALTIMORE, Md. -- UPDATE: Baltimore Mayor Stephanie Rawlings-Blake urged protesters to remain peaceful after a mistrial was declared in the case of Officer William Porter.
"We will not -- and cannot -- be defined by the unrest of last spring. As a community, as a city, we are stronger, and we are united to be better than what some displayed to the world last spring," Rawlings-Blake told reporters Wednesday.
Baltimore Police Commissioner Kevin Davis said officers will respect protesters, provided they remain peaceful.
"Protesters who are lawfully assembled have a friend in the Baltimore Police Department. ... Folks who choose to commit crimes and hurt people and break things and harm people are no longer protesters," he said. "You lose your ability to call yourself a protester when you choose to harm people and destroy property."
Asked for his response to a mistrial being declared, Davis said it was "part of the process."
"I think we all have to respect the process," Davis said. "The process is ongoing. It's not the last time we'll talk about it. And I think we just have to be consistent, measured and thoughtful as we go forward."
Porter remains suspended without pay, he said.
A judge declared a mistrial Wednesday in the case of Baltimore police Officer William Porter after jurors said they were deadlocked and unable to reach a unanimous verdict on any of the four charges against him.
Porter was one of six officers charged in the death of Freddie Gray. Prosecutors will decide whether to retry the case.
The judge told the prosecution and defense attorneys to appear in administrative court Thursday morning to set a new trial date.
A defense attorney declined to comment, saying he was subject to a court gag order.
Gray's family thanked jurors for their service and asked the public to remain calm.
Reading from a statement, Richard Shipley, Gray's stepfather, told CNN he is hopeful Porter will be retried.
"We thank this hard-working jury for their service to the public, their quest for justice, their personal sacrifice of their time and effort. We are not at all upset with them, neither should the public be upset. They did the best that they could," he said.
"We are hopeful that (Baltimore State's Attorney Marilyn) Mosby will retry Officer Porter as soon as possible, and that his next jury will reach a verdict. Once again, we ask the public to remain calm and patient, because we are confident there will be another trial with a different jury. We are calm. You should be calm, too."
Baltimore State's Attorney Marilyn Mosby was in court when the mistrial was declared and looked visibly upset. Prosecutors appeared exasperated.
"The hung jury is, I think, a major setback for the state," said Andrew Alperstein, a Baltimore defense attorney and former prosecutor. But it's no surprise the jury wasn't able to reach a verdict, he said.
"The issues in this case were confusing," he said, "and the evidence against this particular officer was unique, and it was different than the other defendants."
CNN legal analyst Paul Callan said it was surprising that prosecutors decided to start with Porter's case.
"I don't think you want to start out with a problem case, and this case looks to be a very weak case," he said, "and obviously, if there's a hung jury, it was presented in a very weak way."
Mayor calls for calm
Baltimore Mayor Stephanie Rawlings-Blake issued a statement calling for calm after the mistrial was declared.
"This is our American system of justice," the mayor said in a statement posted on her official Twitter account. "Twelve Baltimore residents listened to the evidence presented and were unable to render a unanimous decision."
"As a unified city, we must respect the outcome of the judicial process. In the coming days, if some choose to demonstrate peacefully to express their opinion, that is their constitutional right. I urge everyone to remember that collectively, our reaction needs to be one of respect for our neighborhoods, and for the residents and businesses of our city.
"In the case of any disturbance in the city, we are prepared to respond. We will protect our neighborhoods, our businesses and the people of our city."
Tessa Hill-Aston, president of the Baltimore chapter of the NAACP, said she expected the group of protesters outside the courthouse and elsewhere in the city would grow.
"It's very sad," she told CNN, "because I think that everybody was very happy that police got indicted, and not to get a conviction is painful. We wanted victory in the sense that officers can't get away with killing someone."
With a verdict possible this week, the city of Baltimore said it activated its emergency operations center Monday "out of an abundance of caution."
Baltimore police canceled leave for officers who had days off from Monday through Friday. Officers will be scheduled to work 12-hour shifts instead of the usual 10 hours.
Involuntary manslaughter among charges officer faces
Gray's injury happened as he was being transported following an April 12 arrest. Prosecutors say Porter, one of three black officers charged in the case, was summoned by the van's driver to check on Gray during stops on the way to a police station.
Prosecutors say he should have called a medic for Gray sooner than one was eventually called and also should have ensured that Gray was wearing a seat belt.
Porter was charged with involuntary manslaughter, second-degree assault, reckless endangerment and misconduct in office.
For convictions on some or all of the first three charges, he would face no more than 10 years in prison combined. There is no statutory maximum sentence for the fourth charge, misconduct.
All six officers have been suspended.
Prosecution: It only takes a click and a call
During Monday's closing arguments, prosecutor Janice Bledsoe argued that any officer in Porter's situation would have called for medical assistance once Gray complained.
"'I need a medic.' How long does that take?" the prosecutor asked. "How long does it take to click a seat belt and ask for a medic? Is two, three, maybe four seconds worth a life? That's all it would have taken."
Prosecutors also argued that Porter could have prevented the injury by ensuring Gray was wearing a seat belt while he was in the van.
But during his four-hour testimony last week, Porter said that of the roughly 150 prisoners he has placed in police wagons since joining the Baltimore Police Department in 2010, none was secured with a seat belt -- partly out of concern for officers' safety while in the wagon's tight quarters.
Prisoners were never secured with seat belts during field training, and though cadets were instructed to secure prisoners with seat belts, they were not shown how, Porter said.
During his closing arguments, defense attorney William Murtha said the prosecution's case was full of holes, and that the law requires them to reach a verdict based on the "standard of a reasonable police officer."
"There is an absolute absence of evidence that officer Porter acted in an unreasonable manner," he said.
Testimony in the trial began December 2.