Bill Murphy, an attorney for Gray's family, gave a similar assessment to reporters.
"This hung jury does not mean it's the end of Officer Porter's case," he said.
But what happens next is anyone's guess.
Porter was one of six officers charged in connection with the death of Gray, a 25-year-old who died after sustaining a neck injury while in police custody. Prosecutors will decide whether to retry the case.
Judge Barry Williams will hold one or more scheduling conferences with the prosecution and defense in his chambers -- behind closed doors -- to discuss a new trial date, the court's communications office said.
"Once a new date is selected, the media will be advised of the next steps," the court said.
Gray's family: 'We are calm; you should be calm, too'
A defense attorney declined to comment, saying he was subject to a court gag order. Porter told CNN he was doing well but couldn't comment on the case.
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."
The police union said Porter and his attorneys will continue to press for his acquittal.
"When Officer Porter began this journey through the judicial process, we asked that everyone allow him his day in court as is promised to all citizens. Today, seven months later, Officer Porter is no closer to a resolution than he was at that time," Baltimore Fraternal Order of Police Lodge #3 President Gene Ryan said in a written statement. "Our legal system, however, allows for outcomes of this nature, and we must respect the decision of the jury, despite the fact that it is obviously frustrating to everyone involved."
'A game changer'
was in court when the mistrial was declared and looked visibly upset. Prosecutors, who'd planned to use testimony from Porter in their cases against the other officers, appeared exasperated.
"They didn't offer him immunity. So now, they are in a situation where they have this charged defendant hanging out there, and they can't force him to testify against these other officers," CNN legal analyst Sunny Hostin said. "And so, I think that this is, in many respects, a game changer for this prosecution."
Prosecutors are in "serious trouble," CNN senior legal analyst Jeffrey Toobin said.
"A hung jury is a defeat for the prosecution, especially when they needed Porter to make some of these other cases," Toobin said. "Now, it's not impossible that at least some of these other cases can go to trial without Porter, but his testimony was going to be important."
Murphy, the Gray family's attorney, stressed that the hung jury doesn't mean the case is closed.
"I don't buy the nonsense that this is somehow a victory for either side. It's not," he told reporters. "It's just a bump on the road to justice, and you know, the road to justice has lot of bumps."
But prosecutors now have major strategic decisions to make, Toobin said, like whether to keep prosecuting Porter or offer him immunity.
They haven't spoken to reporters since court adjourned Wednesday.
Mayor calls for calm
Protesters marched in several parts of the city Wednesday night. One group of demonstrators repeated the "Serenity Prayer" as they moved through the streets.
Another group of protesters called police officers racist, chanting, "No justice, no peace" and, "All night, all day, we will fight for Freddie Gray."
The death of Gray, who was black, ignited a wave of protests as debate surged nationwide over whether police use excessive force, particularly against African-Americans.
Riots erupted in Baltimore
in April after Gray's funeral.
Baltimore Mayor Stephanie Rawlings-Blake issued a statement calling for calm after the mistrial was declared.
"We will not -- and cannot -- be defined by the unrest of last spring," she told reporters Wednesday. "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."
Baltimore Police Commissioner Kevin Davis said officers will respect protests, provided they don't turn violent.
"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."
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."
As the possibility of a verdict loomed 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 have been 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.
Jurors said Wednesday that they couldn't reach a unanimous decision on any of the charges against Porter.
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. Porter remains suspended without pay, Davis said Wednesday.
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," he 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."
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.