Edward Brown speaks at a protest outside City Hall about Freddie Gray in Baltimore, Monday, April 20, 2015. (Amy Davis/AP)

ONCE AGAIN, a black man has died, unaccountably, at the hands of police in an American city. Once again, indignant citizens are left to ponder cellphone video and witness accounts, and wonder how this can continue to happen.

Freddie Gray, 25, died Sunday after his spinal cord was nearly severed, a week earlier, while in the custody of Baltimore police. The charging document in Mr. Gray’s case stated that he was “arrested without force or incident” after being run down and tackled by police from whom he fled on foot.

Without incident? Spinal cords are not mostly severed in routine interactions with police officers. Spinal cords are not severed by the use of reasonable force. Spinal cords are not severed simply because a suspect is placed in the back of a police van for a 30-minute ride, as Mr. Gray was upon being taken into custody.

A video taken by a witness to Mr. Gray’s arrest records his screams as he was taken into custody by the officers. The police say he repeatedly requested medical attention while being transported, but, as the Baltimore Sun reported, officers did not call paramedics until the end of the van ride, by which time he was badly injured and unresponsive.

Officials have moved swiftly to release some information, including the names of the six officers under investigation in the incident. The police department has suspended the officers from duty (with pay) and pledged to turn over the results of its investigation to prosecutors by May 1, which would be an exceptionally fast turnaround. On Tuesday, the Justice Department began looking into possible civil rights violations in the case.

The speed of the official response in Baltimore is promising, as are pledges of a transparent and thorough investigation from the mayor and other city leaders. No doubt, the authorities, having seen in Ferguson, Mo., and elsewhere that a community’s fury can boil over into violence, are eager to avoid such an outcome. Mayor Stephanie Rawlings-Blake, recognizing the outrage of Baltimore residents, appealed for calm, acknowledged what she called “a very, very tense time” for the city, and said she is “angry” and “frustrated” while admitting that “we don’t have all the answers.”

The mayor’s empathy was timely and well expressed, but the measure of justice in Baltimore will be whether police and prosecutors can determine precisely what happened during the arrest and in the back of that police van and respond appropriately. The sooner they do so, the better.