Protesters against police violence march towards the White House in Washington, April 29, 2015. [Photo/Agencies]

WASHINGTON - Freddie Gray, whose death after being injured in Baltimore police custody sparked rioting in the city, was "banging against the walls" of a police van and "was intentionally trying to injure himself," a prisoner being transported with Gray has said, the Washington Post reported on Wednesday.

The prisoner's account was contained in an application for a search warrant that was sealed by the court, the Post said. The prisoner, who is currently in jail, was separated from Gray by a metal partition and could not see him, the paper said.

Gray, 25, was found unconscious in the van when it arrived at a police station on April 12. He had suffered a spinal injury and died a week later.

The Post said the affidavit was part of a search warrant seeking the seizure of the uniform worn by one of the officers involved in the arrest or transportation of Gray.