The judge sent the jury back to the deliberation room Tuesday afternoon after jurors said they'd been unable to reach a unanimous verdict. Deliberations continued for about two hours and are set to resume on Wednesday morning.
Porter is the first of six police officers to be tried in Freddie Gray's death from a neck injury sustained while in police custody.
The jury began deliberating Monday, 12 days after testimony began. They've deliberated for nearly 11 hours so far.
If the trial ends with a hung jury, prosecutors would have the opportunity to try the case again.
"This would be a game changer," CNN legal analyst Sunny Hostin said.
A hung jury may result in a decision to change the venue of the trial, she said, and could affect the cases of the other five officers who have yet to be tried.
Defense moves for mistrial
Earlier Tuesday, defense attorneys for Porter moved for a mistrial, citing a letter about the case that Baltimore City Public Schools sent to parents a day earlier.
The defense argued that the nonsequestered jurors could have received Monday's letter, in which school district CEO Gregory Thornton addressed the possibility of civil unrest after a verdict is reached
and the school system's preparations for the verdict.
The defense also asked that the trial's venue be changed. The judge rejected the requests.
Involuntary manslaughter among charges officer faces
Authorities say Gray, a 25-year-old black man, broke his neck on April 12 while being transported in a police van, shackled but not wearing a seat belt. His death a week later sparked demonstrations
and made him a symbol of the black community's distrust of police.
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. They 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 is 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.
Ready for unrest
With a verdict possible this week, the city of Baltimore -- which witnessed protests and unrest after Gray's death
-- said it activated its emergency operations center Monday "out of an abundance of caution."
Last week, Baltimore Mayor Stephanie Rawlings-Blake and Police Commissioner Kevin Davis cautioned the city to be respectful as the verdict gets closer.
"Whatever the jury decides, we must all respect the process," the mayor said last week. "If some choose to demonstrate to express their opinion, that is their right, and we respect that right, and we will fight to protect it. But all of us today agree that the unrest from last spring is not acceptable."
Davis sent a letter to the police force on Monday, saying, "Regardless of the outcome of this trial or any future trial, we refuse to surrender to the low expectations of those who wish to see us fail. ... We serve because we know so many good and decent Baltimoreans need us to stand in between them and crime, disorder, and chaos."
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.
All six officers are being tried separately and consecutively. Next up is the van driver, Caesar Goodson, whose trial is set to begin next month. Goodson, a black officer who is the lead defendant in the indictment, is charged with the most serious offense -- second-degree murder with a depraved heart.
Prosecutors say Gray was arrested April 12 after two police officers on bike patrol made eye contact with him and he took off running.
Officers caught up to him, and they found a knife in his pants. Gray was arrested on a weapons charge, though prosecutors say the knife was a type allowed by Maryland law and police had no reason to detain him.
Porter was not involved until, authorities say, he arrived to look for a possible second suspect and help with crowd control while Gray was put into the van. Gray was unresponsive by the time the van arrived at a police station after making several stops in between.
Authorities say Porter was present at five of the six stops. Among the main issues in the trial: Porter says Gray asked to be taken to a hospital during the fourth stop, but the prosecution and the defense disagree about whether Gray was injured at that point, and about the appropriateness of Porter's decision to not call a medic then.
Dr. Carol Allan, the assistant state medical examiner, testified last week that Gray probably received his neck injury before the fourth stop, most likely at some point when the van stopped suddenly.
But two physicians testifying for the defense said Gray's death likely happened after the fourth stop. The reason, they said, is that Gray would have been unable to speak at that stop had the injury happened earlier.
Porter conceded that Gray asked for medical help during stop four, but said he did not call a medic because Gray didn't appear to be injured and didn't articulate what was wrong. Porter said he helped Gray sit up on a van bench, with Gray "supporting his own head."
Porter testified he told the driver, Goodson, that Gray had asked to go to a hospital, and Porter emphasized in court that Gray was in Goodson's custody.
'How long does it take to click a seat belt?'
Prosecutors also argued that Porter should have ensured Gray was wearing a seat belt.
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."
Using video clips of Porter's statements and video from the scene, prosecutors argued that Porter knew that Gray was too injured to be booked into jail, but did nothing.
"You are looking into the wagon and then you turn your back on Freddie Gray," Bledsoe said.
'An absolute absence of evidence'
During his closing arguments, defense attorney William Murtha said the prosecution's case was full of holes, citing "the absence of real evidence."
"The state is asking you to insert facts into the blanks that don't exist," he told jurors.
Murtha told the jury 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.
During his four-hour testimony Wednesday
, 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 -- in part, 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.
The jury is made up of three black men, four black women, three white women and two white men.