From Washington City Paper:
According to an eyewitness, a D.C. Police detective (pictured above w/ gun) went nuts after kids pelted his Hummer with snowballs at 14th and U Streets NW this afternoon. The veteran detective got out of his car and eventually grabbed for his gun, displaying it to the crowd. He did not immediately identify himself as a police officer. He calmed down once his fellow uniformed cop arrived. Apparently, someone called 911 to report a man with gun. The snowball fight had been well hyped on Twitter. The news of the incident first broke there.