Officers stand watch at the intersection of West North Avenue and Pennsylvania Avenue as protesters walk for Freddie Gray in Baltimore in April. Gray died from spinal injuries about a week after he was arrested and transported in a police van. Jabin Botsford/The Washington Post/Getty Images hide caption

itoggle caption Jabin Botsford/The Washington Post/Getty Images

This combination of undated photos released the Mississippi Bureau of Investigation shows, Marvin Banks, left, and his brother Curtis Banks. The brothers have been arrested in connection with the fatal shooting of two Hattiesburg, Miss., police officers on Saturday. AP hide caption

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Public memorials, like the one at the scene where Freddie Gray was arrested, have become sites to commemorate other deaths of unarmed black men in similar police encounters across the country. David Goldman/AP hide caption

itoggle caption David Goldman/AP

President Obama is condemning the unrest in Baltimore, saying a handful of "criminals" are taking advantage of the situation following the April 19 death of Freddie Gray. Jacquelyn Martin/AP hide caption

itoggle caption Jacquelyn Martin/AP

Robert Bates (left), a Tulsa County, Okla., reserve deputy, leaves his arraignment Tuesday with his attorney. Bates fatally shot a suspect who was pinned down by officers, raising alarms about volunteer police officers who wear badges and carry guns. Sue Ogrocki/AP hide caption

itoggle caption Sue Ogrocki/AP

Makeshift memorials to Walter Scott sprouted up at the scene of his fatal encounter with Michael Slager, the police officer who shot him in the back as he ran away following a routine traffic stop. David Goldman/AP hide caption

itoggle caption David Goldman/AP

Multiple lawsuits accuse the New York City Police Department of pressuring officers into fulfilling monthly quotes for tickets and arrests, resulting in warrantless stops. The NYPD denies the allegations. Spencer Platt/Getty hide caption

itoggle caption Spencer Platt/Getty

Detective Mark Williams (right) speaks with an officer in Richmond, Va. A decade ago, amid a surge in violent crime, Richmond police were identifying relatively few murder suspects. So the police department refocused its efforts to bring up its "clearance rate." Alex Matzke for NPR hide caption

itoggle caption Alex Matzke for NPR