BALTIMORE - Jurors wrestled over what "evil motive" means Monday as they deliberated the fate of the first of six officers to be tried in the death of Freddie Gray, a black man whose neck was broken in a police van, fueling the "Black Lives Matter" movement.

The panel also puzzled over the meaning of "bad faith" and other terms the judge said they must use to decide whether Officer William Porter is guilty of misconduct in office, the least of the charges against him. Porter also is accused of manslaughter, assault, and reckless endangerment, and faces up to about 25 years in prison if convicted of them all.

The verdict will likely set the tone for the others as well as for the city, still healing from an April riot triggered by Gray's death that caused millions of dollars in property damage and exposed the deep divide between the police and the people of Baltimore.

Authorities sought to prevent more trouble ahead of the verdict, opening an emergency operations center Monday and urging parents to control their children. A letter Baltimore City Public Schools CEO Gregory Thornton sent home with students warned that "student walkouts, vandalism, civil disorder, and any form of violence are not acceptable."

"Whatever the verdict, we need everyone in our city to respect the judicial process," Mayor Stephanie Rawlings-Blake said. "We need everyone visiting our city to respect Baltimore."

Prosecutors have had to prove criminality by inaction - that Porter abused his power by failing to save Gray's life.

His defense countered that the case is based on conjecture, and there is no evidence Porter caused Gray's death.

Porter testified that Gray showed no signs of pain or distress before he arrived at the police station critically injured.

Prosecutors said this was a blatant lie.

"Freddie Gray went into the van healthy and he came out of the van dead," prosecutor Janice Bledsoe reminded jurors.

The police van "became his casket on wheels" after Porter repeatedly denied Gray medical care and left him handcuffed and shackled but unbuckled, thus unable to keep his body from slamming into the end of the metal compartment if the van stopped suddenly, Bledsoe said.

Gray was arrested while fleeing police in his neighborhood, just seven city blocks from the station, yet police stopped the van repeatedly during a circuitous trip around West Baltimore that stretched on for 45 minutes.

"Click," she said, and then repeated. "How long does that take, to click a seat belt and click a radio and ask for a medic? Is two, three, four seconds worth a life? It's all it would have taken."