The 38-year-old Cummings was fatally shot at about 1:30 a.m. Sunday while trying to stop a fight that took place in the 1700 block of 29th Street S, police said.
Investigators attributed the arrest to help from the community and thanked those who contacted the agency with information. No other details about the case were revealed by police, who said the investigation is continuing.
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According to an arrest report, Jermaine Cummings was trying to break up a fight early Sunday when he was shot.