BALTIMORE - Jurors in the manslaughter trial of a police officer charged in the death of Freddie Gray said Tuesday they were deadlocked, but the judge told them to keep deliberating.

The jurors sent a note to Baltimore Circuit Judge Barry Williams after about nine hours of discussions over two days. It was not clear if they were stuck on one or more of the four charges Officer William Porter faces. He is the first of six officers who will stand trial in Gray's death.

As the jury deliberated, armored vehicles and police were stationed around the city, and officials promised they were prepared for any unrest. Last spring, parts of the city were burned and looted as frustrations grew over the police department's treatment of blacks.

The note about being deadlocked was jurors' fifth of the day. Later in the afternoon, they told the judge they would deliberate until 5:30 p.m. before going home for the evening.

Jurors received a set of external computer speakers to improve the sound quality of recordings in evidence, including Porter's videotaped interview with Baltimore detectives, police radio transmissions on the day Gray was arrested, and cellphone videos.

Porter faces charges of manslaughter, assault, reckless endangerment, and misconduct. If convicted of all charges, he could face 25 years in prison.