Update on Aug. 7: Relatives of the man who was led by rope said he is mentally ill and regularly sleeps on the streets, struggles they said officers should have known about.

The police chief of Galveston, Tex., apologized on Monday night after a photo emerged of two white officers on horseback leading a black suspect in handcuffs down the street with a rope tied to him.

The image was taken by an onlooker on Saturday and was shared on social media, where it has gone viral and sparked outrage toward the Galveston Police Department, especially among African-American people.

Many pointed to the photo’s symbolism, saying it harked back to slavery and the long history of racism and violence by whites against black people in the United States.