Washington (AFP) - Several Baltimore police officers have been suspended after an African American man died of spinal cord injuries in police custody, police said Monday, pledging to thoroughly investigate the incident.

Freddie Gray, 25, who died Sunday, was arrested on April 12 without force and was later charged with possession of a switchblade knife, according to a police report.

Police vowed to look into how Gray died and said they would set up an independent review board to assess the investigation, due to be completed next Friday.

The investigation is focusing on how Gray died after an autopsy confirmed spinal injuries.

"None of his limbs were broken. He did suffer a very tragic injury to his spinal cord, which resulted in his death," said deputy police commissioner Jerry Rodriguez, adding that the officers were suspended.

"What we don't know, and what we need to get to, is how that injury occurred."

Local NBC television affiliate WBAL reported six officers had been suspended.

A video of the arrest shows police restraining Gray on a sidewalk, then dragging him to a police van while he yells in pain.

Rodriguez said Gray had requested an inhaler after he was arrested and that he became "irate" while sitting in a police van.

Paramedics were called 42 minutes after Gray asked for his inhaler, according to a timeline provided by police.

Police confirmed that Gray had requested medical attention, but would not say when.

The case is the latest in a series of deaths that critics say demonstrates officers' racial bias and excessive use of force.

The incident sparked protests in Baltimore, with about 100 local residents and activists gathered outside a local police station Sunday demanding more information.

Baltimore Police Commissioner Anthony Batts said policies regarding transportation of suspects and providing medical attention were being reviewed following Gray's death.

He said the officers did not appear to use unnecessary force during Gray's arrest, but said they would continue to investigate their conduct.

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"If we find a procedure or a process handled incorrectly, we'll hold people accountable for that," Batts told reporters.

- 'Decades of mistrust' -

Baltimore Mayor Stephanie Rawlings-Blake promised to continue police reform, admitting strained relations between officers and the public.

"It has been very difficult to overcome decades of mistrust, but together I believe we have made great strides forward. However, it's still very clear that we have much more work to do," Rawlings-Blake said.

"I'm frustrated not only that we're here but we don't have all of the answers. I want to know why the officers pursued Mr Gray. I want to know if the proper procedures were followed. I want to know what steps need to be taken for accountability."

The lawyer for Gray's family said he was arrested "for committing no crime" and accused police of a coverup.

"We believe the police are keeping the circumstances of Freddie's death secret until they develop a version of events that will absolve them of all responsibility," attorney William Murphy Jr said in a statement to The Baltimore Sun.

Baltimore is the largest city in the state of Maryland, about 35 miles (56 kilometers) northeast of the US capital Washington.

A series of recent killings of unarmed black men by police in the United States has prompted nationwide protests, charges of racism and revived the debate about undue police force.

Earlier this month, white officer Michael Slager was charged with murder after a video showed him fatally shooting 50-year-old African American Walter Scott in the back as he ran away in South Carolina.