By Donna Owens

BALTIMORE (Reuters) - Prosecution and defense lawyers made closing arguments on Wednesday in the manslaughter trial of the highest-ranking Baltimore police officer charged in the death of black detainee Freddie Gray.

A prosecutor said decisions made by Lieutenant Brian Rice, 42, led to Gray's broken neck when he was left unsecured in a police transport van. But his lawyer argued that Rice had made a correct assessment in a few seconds not to seat-belt him.

Rice is the fourth of six officers to be tried in Baltimore City Circuit Court for Gray's death in April 2015, and there have been no convictions. Judge Barry Williams, who is hearing the case in a bench trial, will issue his decision on Monday.

Gray's death triggered protests and rioting and stoked a national debate about police treatment of minorities. That debate flared anew this month with the deaths of black men at the hands of police in Minnesota and Louisiana.

Prosecutor Janice Bledsoe said in her final pitch that Rice committed an "intentional act that formed a chain (of events) leading to the death of Freddie Gray."

Using a PowerPoint presentation and video footage, Bledsoe told a packed courtroom that Rice had ordered Gray, 25, chased by other officers.

He also had Gray's legs shackled before helping load him into the van on his stomach and failed to secure him with a seat belt, she said.

"Had Lieutenant Rice taken one small measure of compassion and humanity Freddie Gray would be alive," Bledsoe told Williams.

Defense lawyer Michael Belsky said Rice had nine seconds to weigh the situation while factoring in Gray's combativeness, what he called a hostile crowd and the risks of getting in a van to secure a detainee.

"It was professional, correct, it was right and it was reasonable," Belsky said.

He asked rhetorically, "What could he have done?" Summing up the prosecution case, Williams interjected, "The answer is he could have seat-belted him."

Rice had ordered officers to pursue Gray when he fled unprovoked in a high-crime area.

The officer is charged with involuntary manslaughter, misconduct in office and reckless endangerment. Prosecutors dropped another misconduct count, and Williams dismissed a charge of second-degree assault.

If convicted, Rice could face at least 15 years in prison. Williams has acquitted two officers, and the trial of a third officer ended in a hung jury.

(Writing by Ian Simpson in Washington; Editing by Alistair Bell)