Last week three officers set a police dog on a Black man and did nothing as he cried out.
Police confirmed the dog repeatedly bit the man, but said he was resisting arrest.
The St. Louis county prosecutor and the FBI have since opened investigations.
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The FBI is investigating an arrest in which three white police officers in Missouri let a police dog repeatedly bite a Black man.
Woodson Terrace Police Chief Randy Halstead told the St. Louis Post-Dispatch that his department was "fully cooperating" with the investigations being conducted by the FBI and the St. Louis County prosecutor's office.
The moment was captured on video by a bystander, and police later confirmed in a statement that the dog had repeatedly bitten the man.
The video shows the man being arrested on September 20, with police officers leaning him over a car while another officer stands with a dog.
The officer then lets the dog go toward the man, and the dog bites him as the man grabs his ankles and says "help." The officers do not intervene
The officers then push the man to the ground, and the dog pulls his leg. The officers pull the dog back, and then releasing it again, before eventually removing the dog from the man.
Watch the footage here. Readers may find it disturbing.
The City of Woodson Terrace Police Department said in a statement after the arrest that the man was under the influence of "a narcotic," and that he resisted arrest and threatened officers.
The video does not appear to show the man resisting arrest.
The police also said that a local business had accused the man of trespassing, and that he threatened to kill police officers as they walked towards him.
The man has not been charged with any crimes, NBC News reported.
The St. Louis County Prosecutor, which has also opened an investigation into the arrest, and federal officials declined to comment to the St. Louis Post-Dispatch.