There are several legal orders that decide whether a federal agency may hire a non-U.S. citizen

Article Title: Federal Employment of non-citizens

Article Snip: "Hire a non-U.S. citizen into the excepted service or Senior Executive Service, if the annual Appropriations Act, the Immigration Law and the agency’s internal policies allow it...Request approval to hire a non-U.S. citizen into the competitive service, if no qualified U.S. citizen is available. ...Immigration law requirements on employing citizens and aliens...This job is open to section will tell you whether or not a non-U.S. citizen is eligible for the job."

Article Reference: www.usajobs.gov