Like Gray, Porter is black, as are two of the other Baltimore officers charged. The other three officers are white. All six maintain their innocence in connection with Gray's death.
Porter was the first to stand trial -- putting his fate in the hands of a jury made up of three black men, four black women, three white women and two white men.
Talking to CNN on Wednesday after news of the hung jury, he said simply, "I'm doing well," but declined to comment further.
Will Porter stand trial again?
Legal analysts described the mistrial as a major setback for the prosecution
that could affect its efforts to convict the other five officers set to stand trial. The analysts said Porter -- who prosecutors said failed Gray, including by not making sure he wore a seat belt and not calling a medic sooner -- could have testified in those other cases or his statements in his own trial could have been used against the other officers.
Now, though, everything is up in the air.
One main order of business is determining when, or if, Porter will stand trial again. A new trial date could be determined at one or more scheduling conferences Judge Barry Williams may hold with the prosecution and defense in his chambers, the court's communications office said.
Lawyers from the defense and prosecution were in court Thursday morning, but they appeared to finish their business within about half an hour, while Judge Williams moved on to preside over an unrelated trial.
No new court date for Porter had been set of midday Thursday, according to court spokeswoman Terri Charles. She said additional conferences may be held in the coming days.
A defense attorney wouldn't comment about the mistrial, saying he was subject to a court gag order. Prosecutors have not talked to reporters either since leaving court.
In a brief conversation Wednesday evening with The Baltimore Sun, Porter alluded to why: "It's not over yet."
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.
Reading from a statement, Richard Shipley, Gray's stepfather, thanked "this hard-working jury" and said, "We are not at all upset with them, neither should the public be upset. They did the best they could."
Shipley told CNN he is hopeful Porter will be retried "as soon as possible and that his next jury will reach a verdict." In the the meantime, he urged the public to be "calm and patient."
"We are confident there will be another trial with a different jury," he said. "We are calm; you should be calm, too."
The police union said Porter and his attorneys will continue to press for his acquittal.
"Seven months later, Officer Porter is no closer to a resolution than he was at that time," Baltimore Fraternal Order of Police Lodge No. 3 President Gene Ryan said.
"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'
Baltimore State's Attorney Marilyn Mosby
was in court when the mistrial was declared and looked visibly upset. Prosecutors, who had planned to use testimony from Porter in their cases against the other officers, appeared exasperated.
Prosecutors now have major strategic decisions to make. One is whether to put Porter on trial again, which prosecutors sometimes are reluctant to do because the defense has had access to the whole case, CNN legal analyst Sunny Hostin said.
Another question is whether Porter is offered immunity to testify in the trials of the five other officers. The prosecution could ask to put Porter back on the stand anyway as a witness, even if his own legal case is in limbo -- though Hostin noted "they can't force him to testify against these other officers."
"All of these issues will have to be debated," said CNN senior legal analyst Jeffrey Toobin, who characterized the hung jury as "a defeat for the prosecution."
"The prosecution here is in serious trouble," Toobin said.
'A bump on the road to justice'
Porter was picked to be the first to stand trial at least in part because prosecutors wanted to use him as a witness in other cases, Baltimore defense attorney and former prosecutor Andrew Alperstein said.
Had he been convicted, the state could have tried to "flip" him to testify against the other officers, according to Alperstein. Had he been acquitted, he could have been "forced to testify" as what's called "a compellable witness."
"This hung jury is ... a major setback for the state because they have neither option available," Alperstein said.
Yet Murphy, the Gray family's attorney, stressed that the hung jury doesn't mean Porter's own 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."
City heeds calls for calm
As they did in the immediate aftermath of Gray's death, protesters marched in several parts of Baltimore on Wednesday night.
One group repeated the "Serenity Prayer" as demonstrators moved through the streets. Another group called police officers racist, chanting, "No justice, no peace" and, "All night, all day, we will fight for Freddie Gray."
Baltimore Mayor Stephanie Rawlings-Blake issued a statement calling for calm after the mistrial.
"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."
Involuntary manslaughter among charges officer faces
Gray's injury happened as he was being transported following his arrest. Prosecutors said Porter was summoned by the police van's driver to check on Gray during stops on the way to a police station.
Prosecutors said 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, Baltimore Police Commissioner Kevin Davis said Wednesday.
Asked for his response to a mistrial being declared, Davis said it was "part of the process."
"The process is ongoing," he said. "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."