Gray, 25, suffered a severe spinal cord injury in the back of a police transport van in April and died a week later. Protests against police brutality broke out in Baltimore because of the case, and rioting, looting and arson occurred on the day of Gray's funeral. The case has attracted international attention.
On May 1, six officers involved in his arrest and transport were criminally charged by Baltimore State's Attorney Marilyn Mosby, and later indicted.
The six officers' trials were severed, and scheduled to proceed consecutively and quickly through the lower court.
How did we get here?
Porter was the first officer to go to trial in December. The proceedings ended in a mistrial after the 12-member jury failed to reach a consensus on any of the four charges against him. A retrial was scheduled for this summer.
Attorneys for the other officers have also objected to Porter being forced to testify.