WASHINGTON D.C., June 30, 2016 (Xinhua) -- U.S. Homeland Security chief Jeh Johnson testifies before the Senate Committee on the Judiciary during a hearing on Capitol Hill in Washington D.C., capital of the United States, June 30, 2016. U.S. Homeland Security chief Jeh Johnson said on Thursday enhanced security presence were expected at U.S. airports and other domestic transit centers over the July 4 weekend after the deadly Istanbul terror attack. (Xinhua/Bao Dandan)