A police officer awaiting trial has been called to testify against his colleague, a van driver charged with murder in the death of a 25-year-old black man in police custody.

Officer William Porter, whose trial ended in a mistrial in December and is scheduled for retrial in September, is the state's 12th witness against Caesar Goodson, who faces second-degree murder, manslaughter and other charges stemming from the death of Freddie Gray.

Before Porter was called to the stand his attorney, Joseph Murtha, objected to an appeals court decision forcing officers in the Gray case to testify against each other. Baltimore Circuit Judge Barry Williams denied the objection.

Gray died April 19, 2015, a week after his neck was broken in the back of Goodson's police van.

Prosecutors say Goodson gave Gray a "rough ride" and was negligent when he failed to get the man medical attention. Goodson's attorneys say the officer acted reasonably and did nothing wrong.