From News 9 Oklahoma City:

During the chase the two officers became separated. Eller found himself alone when he caught up to Jermaine in a driveway of a nearby home and as he tried to place him under arrest, a fight ensued. During the struggle, Jermaine was able to take Eller’s police baton and then proceeded to strike him over the head somewhere between six and 12 times.

According to a report, that’s when a witness nearby charged up with his weapon drawn and told Jermaine he would shoot him if he did not stop hitting Eller. That heroic witness has not been identified.