Gray, 25, died a week after he suffered a spinal injury while handcuffed but unrestrained in a police van last April.
Porter was the first of six officers tried in court for the incident. His trial ended in a hung jury, but a retrial is scheduled for later this year.
In the meantime, the prosecutor wants him take the stand against fellow officers, but Porter refused.
The prosecutor says Porter is an invaluable witness in its case against the other officers.
According to USA Today, Porter said at his trial that he accompanied Officer Caesar Goodson, who was driving the van, during most of the trip to the police station. Porter testified that Goodson declined to take Gray to the hospital when he asked for medical assistance. Instead, Goodson stopped to pick up another prisoner.
CBS notes that Porter had key interactions with other officers involved in the case:
The court issued two rulings. The first ruling agreed with Baltimore Circuit Judge Barry Williams’ decision to force Porter to testify against Sgt. Alicia White and Officer Caesar Goodson, all of whom face manslaughter, assault, reckless endangerment and misconduct in office charges. Goodson, the van driver, faces the most serious charge, second-degree murder.
A second order reversed Williams’ decision that Porter did not have to testify against officers Garrett Miller, Edward Nero and Lt. Brian Rice. They face misconduct in office, reckless endangerment and assault charges. Rice also faces a manslaughter charge.
SOURCE: CBS News | PHOTO CREDIT: Getty | VIDEO SOURCE: Inform