Five Arizona police officers who were shown on camera pummelling an unarmed man last May will not face any charges, authorities have announced.
The beating of the 33-year-old man by Mesa Police Department officers was legally justified, investigators from another police force determined.
A lawyer for Robert Johnson says the attack was not legally permitted and threatened to sue in civil court.
Video shows Mr Johnson being hit repeatedly in the face and body.
The ruling was made by the Scottsdale Police Department, which was asked by the Mesa police - 20 miles (32km) east of Phoenix - to investigate the incident.
Detectives reviewed eight body cameras worn by the officers, as well as the CCTV, consisting of over two hours of footage.
Scottsdale investigators determined on Monday that "no criminal charges are warranted against the involved officers as the use of force was legally authorised and justified under Arizona State Law".
They added that the Maricopa County Attorney's Office agrees with the ruling.
Police were called on 23 May after a woman reported her ex-boyfriend was trying to break into her flat. Mr Johnson was at the scene with the ex-boyfriend.
Officers confronted Mr Johnson on the third floor of the apartment complex, where he lives.
Mr Johnson was accused of disorderly conduct and hindering police, but those charges were dropped after video of his arrest went viral.
His lawyer, Benjamin Taylor, said: "The video flatly contradicts the self-serving accounts of the police officers involved.
"The use of government violence against co-operating citizens, who have committed no crime and who pose no threat, is a crime without justification or legal authorisation."
Mesa police say the officers involved remain on administrative leave, pending an internal department review.
A police report on the day of the attack said Mr Johnson was "verbally defiant and confrontational" and refused to sit when officers instructed him to do so.
It is one of several recent cases in which Mesa police have been accused of using excessive force.
Earlier this month, the Maricopa County Attorney's Office announced charges would not be filed against Mesa officers in another incident involving a teenage robbery suspect.
Police body camera showed the 15-year-old boy was roughly manhandled in police custody.