The department's acknowledgement came after a week of scrutiny and near-daily demonstrations over what protesters say is police mistreatment of blacks in Baltimore and throughout the country. Fierce debate has been stoked by the deaths of unarmed black men in Ferguson, Missouri, and New York City.
Gray was taken into custody April 12 and during his arrest or in the van suffered a spinal injury. Six officers have been suspended with pay.
Commissioner Anthony Batts said it was possible Gray was injured before the van ride, but also possible he had a "rough ride" - where officers hit the brakes and take sharp turns to injure suspects in the back of vans.
Gray was arrested after he made eye contact with officers and ran away, police said.
Gray was not buckled in, a violation of police policy.
Gray asked for medical help several times, and after a 30-minute ride that included three stops, paramedics were called.
"He was not buckled in the transportation wagon as he should have been. There's no excuse for that, period," Batts said. "We know our police employees failed to give him medical attention in a timely manner multiple times."
Deputy police commissioner Bystander video shows Gray screaming as officers carried him to the van, his legs appearing limp.
Protesters promised their biggest march last night, when they would try to "shut down" the city.
Malik Shabazz of Black Lawyers for Justice, who predicted thousands would turn up, rejected the notion that he was an outside agitator who would stir up trouble.
Mayor Stephanie Rawlings-Blake expects the results of the investigation to be turned over to prosecutors in a week.
The mayor said Gray's possible rough ride was clearly not a one-off.
In 2005, Dondi Johnson died of a fractured spine after he was arrested for urinating in public and transported without a seat belt, with hands cuffed behind him.
