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Officers had been called to the brawl at the junction of Witton Road and Endicott Road, just after 19:45 GMT.
Those involved in the disorder had left, but police discovered evidence that showed a gun had been fired.
Investigators also found Lemathy had been stabbed in the fight and had taken himself to hospital.
The force later recovered CCTV that showed the disorder taking place.
They arrested and questioned Lemathy after he was discharged from hospital.