A Pennsylvania man sentenced to 44 months in prison for making threats on Facebook—threats he said were emulating the rapper Eminem—won a new trial Monday after a 7-2 US Supreme Court ruling overturned his conviction.

The case of 31-year-old Anthony Elonis raises the question of when an online threat becomes fodder for criminal prosecution under a federal threats statute. After all, words online don't always mean what they say.

But the Supreme Court didn't agree on the answer. A majority of justices said that the government must prove that Elonis's online attacks against an elementary school, his estranged wife, and even the FBI were made with the "subjective intent to threaten."

"Elonis’s conviction cannot stand. The jury was instructed that the Government need prove only that a reasonable person would regard Elonis’s communications as threats, and that was error. Federal criminal liability generally does not turn solely on the results of an act without considering the defendant’s mental state," Chief Justice John Roberts wrote (PDF) for the majority.

In dissent, Justices Samuel Alito and Clarence Thomas wrote that the majority's opinion "is certain to cause confusion and serious problems."