WASHINGTON D.C., July 14, 2016 (Xinhua) -- U.S. Homeland Security chief Jeh Johnson testifies before the House Homeland Security Committee on Capitol Hill in Washington D.C., capital of the United States, July 14, 2016. U.S. Homeland Security chief Jeh Johnson warned on Thursday of possible violence during the upcoming Republican National Convention. (Xinhua/Bao Dandan)