A Baltimore police officer testified on Monday that black detainee Freddie Gray asked for help while being transported in a police van, where he suffered a broken neck.

The officer, William Porter, took the stand in the trial of Officer Caesar Goodson, 46, the van's driver. Prosecutors contend Goodson gave Gray a "rough ride" and he is accused of second-degree depraved-heart murder and other charges in Gray's death in April 2015.

His death triggered protests and rioting in Baltimore and stoked a U.S. debate on police treatment of minorities. Goodson is the third of six officers to go on trial, and his attorneys argue that Gray caused his own fatal injuries.