A judge ruled Baltimore Police Officer William Porter must testify in the trials of two other police officers accused in the death of Freddie Gray.
Porter's trial ended with a hung jury last month.
Driver Caesar Goodson faces the most serious charge of the six officers accused in Gray's death and has pleaded not guilty. His charges include manslaughter and second-degree "depraved-heart'' murder.
On Wednesday, Baltimore Circuit Judge Barry Williams compelled Porter to testify in Goodson's trial, which starts next week, and the trial of Sgt. Alicia White, the supervisor the day Gray sustained his fatal injuries. The testimony will be given under a grant of immunity, meaning it couldn't be used against him in court. It's the first time that has happened in Maryland.
"It's almost sacrilegious to make a defendant testify to give him immunity," legal analyst Warren Brown said. "That's why it never occurred."
Porter took the witness stand at Wednesday's hearing but refused to answer questions from prosecutors. He wants to avoid testifying in Goodson's trial, saying he'd invoke his Fifth Amendment right against self-incrimination. Now, that could result in his being held in contempt.
Porter has been subpoenaed as a prosecution witness to testify that he told the van driver Gray needed medical attention and couldn't breathe.
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Porter is charged with manslaughter, assault, reckless endangerment and misconduct in office. His retrial is scheduled for June 13.
Goodson's trial will remain in Baltimore after Williams denied a defense motion to move the proceedings. Williams also ruled that jurors would be anonymous but not sequestered.
Prosecutors said Porter is partly responsible for failing to buckle Gray into a seat belt and for not calling an ambulance when Gray indicated he was in distress. Porter told jurors he didn't think Gray was injured and it was the van driver's responsibility to fasten Gray's seat belt.
Demonstrations were initially peaceful following the 25-year-old's death in April, but in the hours following his funeral, rioters threw bottles and bricks at police, injuring almost 100 officers. More than 200 people were arrested as cars, and businesses burned.