The New Years Eve fireworks was the only scene actually filmed in Baltimore. The snow on the ground was the giveaway since this city has not had that much snow during the holidays in many years.
Of the main cast, only Shailene Woodley (Eleanor) is American. Ben Mendelsohn (Lammark) is Australian, and both Ralph Ineson (Dean) and Jovan Adepo (Mac) are English, though the latter holds dual citizenship in both America and England.
The Baltimore City Police wear a patch on their sleeves that is in color. The police are depicted wearing a black & white patch.