The court's seven judges peppered attorneys for the state and for the six Baltimore police officers charged in Gray's arrest and death with questions about the protections of the state's immunity statute, and whether prosecutors have an absolute right to deploy it as a legal tool.
The judges did not immediately issue a ruling, and it's not clear when they will.
At one point, Judge Clayton Greene Jr. asked Porter's attorney why it's a problem for Porter to testify as a witness, given that he already testified in December at his own trial, which ended in a hung jury.
"Isn't the cat kind of out of the bag now?" Greene asked. "What is the harm to Officer Porter if he were to simply tell his story again in multiple trials?"
But Judge Lynne A. Battaglia said prosecutors face a "minefield" of problems in their retrial of Porter if they push forward with him as a witness in the other trials. An attorney for the state's attorney's office agreed, saying prosecutors will encounter a "heavy burden."
The two-hour hearing took place in the Court of Appeals' wood-paneled courtroom in Annapolis, with attorneys stepping to the podium with prepared arguments — and quickly encountering questions from the red-robed judges.
Among those in attendance was Baltimore State's Attorney Marilyn J. Mosby and her staff, and the six officers charged in the case. The two sides sat on opposing walls, facing each other.
Gray, 25, died last April after being shackled and placed into a police transport van without being secured in a seat belt. The charges against the six officers were celebrated by those concerned that officers rarely face charges when people die in police custody, but many legal observers have said there would be significant hurdles to winning convictions.
Only Porter's case has gone to trial, ending in a mistrial. It was scheduled for retrial this summer. The other cases then landed in the appellate courts.
Chief Judge Mary Ellen Barbera has placed the appeals on an expedited track, but has not indicated when the court might rule. Until the high court rules, the officers' cases cannot proceed in the lower court, where the matter of Porter being forced to testify first arose.
Paul Mark Sandler, a local attorney and longtime editor of a guide to appellate practice in Maryland, said he would expect the judges to take their time in drafting careful opinions in the Gray cases.
"It not only affects these cases. It will affect cases for many, many people and the procedures and how such cases are governed," he said. "It's not just, 'Let's get this case done.' The court has to consider the public interest."
Attorney Jamaal W. Stafford, who served as a clerk for former Chief Judge Robert M. Bell and now has a private practice in Chevy Chase, said the judges conference the same day to discuss the case, then vote.
If there is a strong majority, the court can quickly issue an order stating how it will rule, then release a full opinion at a later date, said Stafford, who is not involved in the Gray case. Such orders are most common in cases where there is "a level of urgency and there absolutely needs to be clarity on which way things will go," Stafford said.
But if the court is heavily split or if some of the judges are undecided about how to vote, the court will hold off issuing an order while opinions are drafted.
The undecided judges will "wait to see what the opinions look like, majority and dissent," and then decide whether to sign on to one or the other or parts of both, Stafford said.
Legal experts have said it is rare for the high court to take up cases at this juncture, before any verdicts have been reached. Typically, trial judges issue a variety of rulings on evidence and arguments, and the parties must wait until the trial has concluded to file an appeal.
Porter has argued that his rights are being violated after he was ordered by Circuit Judge Barry G. Williams to testify against Officer Caesar R. Goodson Jr. and Sgt. Alicia White. The state's second-highest court, the Court of Special Appeals, had previously agreed to hear his case.
Williams had granted Porter a type of immunity intended to prevent his testimony in the Goodson and White cases from being used against him at his retrial.
Part of Thursday's arguments in the high court focused on whether prosecutors could appeal Williams' later decision to not compel Porter to testify against three other officers.
Chief Deputy State's Attorney Michael Schatzow argued that Williams had overstepped his "ministerial role" in blocking prosecutors from calling Porter as a witness in the trials of Officers Edward Nero and Garrett Miller and Lt. Brian Rice.
Williams said at the time that prosecutors, who had not previously said Porter would be a witness in those cases, were trying to stall the trials.
Though prosecutors typically can't appeal a pretrial decision, Schatzow contended that the decision regarding Porter was a separate, concluded civil matter that they could ask the appeals court to review.
The defense attorneys have said there is no precedent in Maryland for prosecutors to appeal such a ruling, or for a defendant facing trial to be forced to testify against other co-defendants.
Porter's attorney, Gary Proctor, said Porter's rights as a defendant facing his own manslaughter charges should trump prosecutors' desire to call him as a witness. Making him testify multiple times in a row would create a "minefield" of possible or apparent contradictions in his testimony.
"It's unlikely," Proctor said, "that anyone can say the same thing five times in lockstep."
Proctor said that while the state may have the burden of proving that they have used nothing from Porter's compelled testimony at his retrial, neither he nor the court could ever "pierce the minds" of the prosecutors to ensure that is the case. Proctor said there is so much attention on the officers' trials that whatever Porter testifies to will be heard by potential jurors in his retrial.
Assistant Attorney General Carrie Williams said that the state has a "heavy burden" of proving that nothing it uses to prosecute Porter at his retrial is based on his immunized testimony.
That it is "uncharted territory speaks volumes about the burden the state faces," she said.
She quoted Robert Louis Stevenson: "Sooner or later, we must all sit down for a banquet of consequences."
But she also said that because Porter has already had a trial, it would be easy to determine whether prosecutors were bringing in new evidence at his second trial.