BALTIMORE (Reuters) - A Maryland appeals court on Monday delayed the murder trial of a Baltimore police officer in the death of black detainee Freddie Gray until it can decide on whether a key witness should testify.

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Caesar Goodson Jr.’s trial was to have begun on Monday in a case that fueled a nationwide debate over race and policing in the United States.

The ruling by the Court of Special Appeals in Annapolis delayed the trial until it can decide whether another William Porter, the Baltimore police officer whose own trial in the Gray case ended in a hung jury in December, could be compelled to testify against Goodson and another officer.

They are among a total of six officers facing trial in Baltimore City Circuit Court for the death of Gray, who died of a broken neck in April after being transported in a police van.

A spokeswoman for Maryland’s court system said motions would have to be filed in Goodson’s case and oral arguments held before the appeals court could rule on whether Porter will testify. There is no time frame for a decision, she said.

Goodson faces the most serious charge, second-degree depraved heart murder, and jury selection had been scheduled to start on Monday.

Goodson, 46, was the driver of the police transport van where Gray, who was arrested after fleeing police, suffered a broken neck. He died a week later.

Baltimore, a majority black city of 620,000 people, exploded in protests and arson after Gray’s death. The unrest followed other police killings of black men in New York and Ferguson, Missouri.

Porter’s trial on involuntary manslaughter and other charges ended in hung jury in December. Prosecutors want him as a witness against Goodson and last week Judge Barry Williams ordered Porter to testify since he had been offered immunity from prosecution for what he might say on the stand.

The Court of Special Appeals agreed on Friday to a motion by Porter’s lawyers to temporarily halt William’s order. [L1N14S1G2]

Porter has said he will invoke his constitutional right against self-incrimination if called to testify. His retrial is scheduled for June.

Goodson, who is black, faces up to 30 years in prison if convicted of the murder charge. In Maryland, depraved heart murder is a killing done while acting with extreme disregard for human life.

He also is accused of manslaughter, three counts of assault, misconduct in office and reckless endangerment.