President Donald Trump on Thursday fired his embattled Homeland Security Secretary Kristi Noem, after mounting criticism over her leadership of the department, including the handling of the administration's immigration crackdown and disaster response. Trump says he'll nominate in Noem's place Oklahoma Republican Sen. Markwayne Mullin. Trump made the announcement on social media Thursday, two days after Noem faced a grilling on Capitol Hill from GOP members as well as Democrats. Trump says he'll make Noem a "Special Envoy for The Shield of the Americas," a new security initiative he says will focus on the Western Hemisphere.