The US Department of Justice has opened a civil rights inquiry into the death of an African-American man whose neck was broken when he was arrested in Baltimore.

Freddie Gray was arrested on 12 April and placed in a police transport van – and was rushed to the hospital in critical condition about 30 minutes later. He died on 19 April of what police described as a significant spinal injury.

The investigation will look for civil rights violations, according to spokeswoman Dena Iverson.

In federal civil rights cases, investigators look for evidence that an officer wilfully violated a person's civil rights by using unreasonable force.

The Baltimore Police Department released the names of six police officers suspended with pay after the arrest of Freddie Gray.

The officers were identified by the department on Tuesday as: Lt. Brian Rice, 41, who joined the department in 1997; Officer Caesar Goodson, 45, who joined in 1999; Sgt. Alicia White, 30, who joined in 2010; Officer William Porter, 25, who joined in 2012; Officer Garrett Miller, 26, who joined in 2012; and Officer Edward Nero, 29, who joined in 2012.

After an "in-custody death," it is standard procedure to release the names of officers involved, said Baltimore Police Department spokesman Capt. Eric Kowalczyk.

The Mayor of Baltimore Stephanie Rawlings-Blake has promised answers as to how Gray died.

"I'm going to make sure that as we get information that we can confirm, we're going to put that information out in the public," Rawlings-Blake told CNN. "I want people to understand that I have no interest in hiding information, holding back information."

Among the questions she wants answered are: Why did police stop Gray in the first place? And why did arresting officers make what the Mayor called the mistake of not immediately requesting medical attention when Gray asked for it?

"He was dragged a bit, but then you see him using his legs to get into the van, so he was able-bodied when he was in the van, and we know that when he was finally taken out of the van, he was unresponsive," she said.

"We will get to the bottom of it, and we will go where the facts lead us," she said. "We will hold people accountable if we find there was wrongdoing."