On January 12, the Supreme Court heard a case involving a federal law that empowers the federal government to keep dangerous people locked up after their prison sentences are over. Enough justices seemed skeptical, however, so the law may be struck down as unconstitutional.

The Supreme Court heard arguments on Tuesday in U.S. v. Comstock. At issue was a federal statute providing that if the government deems a criminal whose prison term is about to end to be a “sexually dangerous” person, then they can continue to keep him confined—potentially for life. This criminal doesn’t need to be in prison for a sex crime; he can be imprisoned for any reason, such as tax evasion.

The issue before the Court was simple: Which provision of the Constitution authorizes the feds to lock up someone for being “dangerous” as part of a civil commitment? A civil commitment proceeding is when a court confines a person for something other than a criminal conviction, such as a local judge having someone committed to a mental institution for being crazy in a manner that endangers themselves and others.

If a person is convicted of committing a federal crime, then the feds can imprison that person. The power to imprison them for committing the crime flows from whatever part of the Constitution authorizes the federal government to make that act illegal in the first place.

But what power do the feds have simply to lock up “dangerous people?”

What makes this case important is it goes to the heart of how the federal government is fundamentally different from state governments. States have what are called governments of general jurisdiction. This includes the police power, which is the power to make laws for public health, safety, welfare and morality.

By contrast, the federal government is a government of limited jurisdiction. That means it only has whatever powers are delegated to it by specific clauses in the Constitution. If no constitutional provision authorizes a particular federal law, then that law is unconstitutional.

The police power authorizes state governments to confine people that pose a danger to public health or safety. That, for example, is why states can put insane people in a mental institution.

But the federal government has no police power, so it can’t have a state-style civil commitment law to lock someone up for things unrelated to federal crimes. It’s just none of the federal government’s business, as far as the Constitution is concerned.