By Donna Owens

BALTIMORE (Reuters) - The Baltimore police van driver who transported black detainee Freddie Gray left him unattended after the ride in which he broke his neck, a police officer testified on Monday at the murder trial of a third officer in Gray's death.

Prosecutors allege that Officer Caesar Goodson Jr., 46, gave Gray a "rough ride" or drove erratically after Gray was arrested in April 2015. Their questioning of Officer William Porter aimed at showing Goodson ignored department procedures and failed to seek medical help.

Gray's death triggered rioting and protests and stoked a U.S. debate on police treatment of minorities. Gray was arrested for fleeing officers unprovoked and was bundled into a police van while shackled and not seatbelted, a violation of procedure.

Goodson is among six officers charged in Gray's death. He is accused of second-degree depraved heart murder and other charges, and his attorneys have argued that Gray caused his own injuries.

During two hours of testimony in Baltimore City Circuit Court, Porter said Goodson was not around after he drove Gray to the Western District station. Gray was unresponsive in the back of the van, he said.

"I don't recall where he was," Porter testified under questioning by defense attorney Matthew Fraling.

He added that Goodson might have been dealing with a second detainee.

Porter was present at some of the stops that Goodson made. Prosecutor Michael Schatzow underscored in his questioning of Porter that leaving a prisoner unattended was a violation of police procedure.

Schatzow read from a statement from Porter in which he said he had told Goodson during Gray's transport that he should be taken to a hospital.

Asked if Goodson had agreed, Porter said, "Sure."

Porter said that he responded to a call to assist Goodson at the fourth of six stops the van made. He found Gray lying on the van's floor, and helped him onto a bench.

A medical examiner's report found that Gray was fatally injured between the second and fourth stops. Judge Barry Williams is deciding the case in a bench trial.

Porter's testimony in a case in which he also faces trial was the result of a legal battle that reached Maryland's highest court.

Porter's trial on a manslaughter charge ended in a hung jury, and he faces retrial in September. Williams acquitted one officer, Edward Nero, in a second trial.

(Writing by Ian Simpson in Washington; Editing by Dan Grebler and Matthew Lewis)