 For NPR News in the Twin Cities, I'm Emily Bright. Police Chief Brian O'Hara kept his promise to crack down on illegal fireworks usage over the Fourth of July. Police in Minneapolis arrested more than two dozen teens and young adults since Thursday on suspicion of shooting fireworks at people and vehicles. O'Hara says most of the trouble this July 4th was in Dinkytown near the University of Minnesota. He says he was on patrol there with a group of officers when someone lobbed a mortar at them.