Story highlights "The nature and the posting of the wording is ... unacceptable," police official says

Tensions remain in Baltimore over the death of Freddie Gray in police custody

(CNN) Baltimore authorities are investigating after photos emerged appearing to show a sign inside the door of a police van that reads, "Enjoy your ride, cuz we sure will!"

Prosecutors say Gray suffered a critical neck injury in April after being handcuffed and shackled in the back of a Baltimore police transport van that made several stops. Six police officers have been indicted on charges connected to his death.

The city's police department was quick to criticize the sign on the van door after the photos came to light Thursday.

"The nature and the posting of the wording is both concerning and unacceptable," Baltimore City Police Sgt. Jarron Jackson said.

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