States hold general police powers over public health and safety, and Congress has certain powers denied to states, such as declaring war. But federal criminal law can relate only to specific powers of the federal government. There is no general federal murder statute, for example, but there are laws against killing a federal officer.
Because the sex crimes that federal prisoners were considered likely to commit in the future fall under state rather than federal law, the appeals court found that Congress lacked authority to detain the inmates.
At the arguments in January, Ms. Kagan said the power to confine such prisoners was implicit in the government’s duty “to run a responsible criminal justice system.” The law aims “to make sure that sexually dangerous, mentally ill people don’t fall through the cracks between federal custody and the re-establishment of state control,” she said.
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