BALTIMORE (Reuters) - A jury is deadlocked in the trial of police officer William Porter, charged in the death of black detainee Freddie Gray, Baltimore City Circuit Court Judge Barry Williams said on Tuesday.

Baltimore Police officer William Porter approaches the court House in Baltimore, Maryland, November 30, 2015. Porter is one of six Baltimore City police officer charged in connection with the death of Freddie Gray. REUTERS/Patrick Samansky/Pool -

Williams said he had received a note from the jury of seven women and five men after it had weighed the fate of Officer William Porter, 26, for almost 10 hours.

Williams ordered the jury to keep trying to reach a verdict. It was not immediately clear how the panel was deadlocked.

Porter is charged with involuntary manslaughter and other charges in Gray’s death from a broken neck sustained in the back of a police van. Gray’s death in April triggered protests and rioting and intensified a U.S. debate on police treatment of minorities.

The jury of seven women and five men started deliberations on Monday. Williams earlier on Tuesday had rejected a request by defense lawyer Gary Proctor to declare a mistrial and order a change of venue.

David Jaros, an associate law professor at the University of Baltimore, said it was not surprising that the panel had been unable to reach a decision, given the complexity of the legal issues.

“I have seen juries deadlocked and come back within 10 minutes, or go for days” without reaching a decision, he said.

Jaros added that if the jury remained deadlocked, Williams could issue a so-called Allen charge, pushing them to reach a verdict.

Proctor had asked for the trial to be moved on the grounds the city is too sensitive to the case. He cited a letter sent from the head of the city’s public schools to students, staff and parents on Monday warning that violence and walk-outs would not be tolerated after a verdict.

Defense attorneys have repeatedly asked that the trial be moved, saying the unrest and publicity tainted prospects for a fair trial.

Porter is the first of six officers to face trial. He also faces charges of assault, endangerment and misconduct. Three of the six officers, including Porter, are black.

Gray, 25, was arrested after fleeing from police. He was put in a transport van, shackled and handcuffed, but was not secured by a seat belt despite department policy to do so.

Gray told Porter he needed medical aid and Porter put him on a van bench. According to testimony, Porter told the van’s driver and a supervisor that Gray had asked for aid but none was summoned.

Baltimore, a black majority city of about 620,000 people, has braced for possible trouble from a verdict.

Baltimore has opened an emergency operations center and police leave has been canceled. Officers from outside Baltimore have been readied to help if needed.

Police Commissioner Kevin Davis sent a letter to officers on Monday saying the department would protect the city at the same time as allowing peaceful protests.

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