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A Baltimore prosecutor announced criminal charges on Friday against six police officers in the arrest of Freddie Gray, whose death after suffering a spinal cord injury in police custody touched off riots.

One officer, the driver of the van that carted Gray away after his arrest, was charged with a count known as second-degree depraved heart murder. Other charges against the six officers included manslaughter, assault and misconduct.

All six officers were booked and released on bond by Friday evening.

Marilyn Mosby, the state's attorney for Baltimore, said that Gray's arrest on April 12 was illegal and had been ruled a homicide.

She said officers failed to get medical help for him even though he repeatedly asked for it. She also said a switchblade that officers accused Gray of carrying illegally was a legal pocket knife.

"I assured his family that no one is above the law and that I would pursue justice on their behalf," Mosby said at a press conference, where cheers went up when she announced the charges.

Gray, 25, died a week after his arrest. His was the latest case in a national debate about police tactics. In Baltimore, dozens of people were arrested during civil unrest on Monday night, including riots, looting and fires.

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Gov. Larry Hogan called for continued calm, and President Barack Obama said it was important for the people of Baltimore to get the truth. "That's what people around the country expect," he said.

Mosby said that Gray repeatedly asked for medical attention during his ride from the scene of his arrest to the booking site.

"A medic was finally called to the scene, where the medic determined that Mr. Gray was in cardiac arrest," she said.

Baltimore's police commissioner said Thursday that his department had handed over confidential information on how Gray died to prosecutors. Meanwhile, the Justice Department is also working on its own independent investigation.

Mosby said in Friday's news conference that her office had been investigating Gray's death independently of police's investigation.

"We knew that this was a serious case," she said. "From Day 1, we investigated. We’re not just relying solely on what we were given by the police department."

The Baltimore police union defended the officers in an open letter to the prosecutor.

"Not one of the officers involved in this tragic situation left home in the morning with the anticipation that someone with whom they interacted would not go home that night," wrote Gene Ryan, a union official.

"As tragic as this situation is, none of the officers involved are responsible for the death of Mr. Gray," he wrote. "To the contrary, at all times, each of the officers diligently balanced their obligations to protect Mr. Gray and discharge their duties to protect the public."

Mosby comes from five generations of police officers. She was sworn in as state's attorney in January after a surprise win in November, and spoke out against police brutality during her campaign.

"Police brutality is completely inexcusable. I'm going to apply justice fairly, even to those who wear a badge," she told the Baltimore Sun.

IN-DEPTH

Jon Schuppe contributed to this report.