The U.S. flag flies in front of the Supreme Court on May 18. (Joshua Roberts/Reuters)

The Supreme Court on Monday made it harder for prosecutors to convict those who make violent statements on Facebook and other social media, saying it is not enough that an ordinary person would find the rants threatening.

In its first examination of the murky rules regarding conduct on the Internet, the court moved cautiously while throwing out the conviction of a Pennsylvania man whose postings, delivered in rap-lyric style, suggested killing his estranged wife, federal law enforcement officials and even a kindergarten class.

Chief Justice John G. Roberts Jr., noting that Anthony Douglas Elonis had said he intended his postings to be fictitious and even therapeutic, said a defendant’s state of mind had to be considered.

But the opinion offered little in the way of specifics about what must be proved for a conviction, and Justices Clarence Thomas and Samuel A. Alito Jr. criticized the opinion as more confusing than enlightening.

“This failure to decide throws everyone from appellate judges to everyday Facebook users into a state of uncertainty,” Thomas wrote in dissent.

The Supreme Court protected some percieved threats on Facebook and other social media, saying prosecutors must prove they were intended as threats. (Chris Ratcliffe/Bloomberg)

The narrow opinion said it was not necessary to address whether the First Amendment’s guarantee of free speech protected Elonis’s Facebook statements. The opinion also declined to take a position on whether it would be enough for a conviction to show that a defendant had been reckless in making inflammatory statements, as Alito proposed.

For the justices, it was sufficient for now, Roberts wrote, to correct a misinterpretation by most lower courts that the poster’s intent is immaterial and what matters only is how the message is received.

Roberts defended the majority’s go-slow approach. “Such prudence is nothing new,” he wrote.

Civil libertarians praised the limited ruling.

Steven R. Shapiro, legal director of the American Civil Liberties Union, said the law “for centuries required the government to prove criminal intent before putting someone in jail. That principle is especially important when a prosecution is based on a defendant’s words.”

He added, “The Internet does not change this long-standing rule.”

Groups battling domestic violence and advocating for victims said they worried the ruling would make it harder to convict those who make threats and said the ease and accessibility of social media have made the problem worse.

“The Internet is the crime scene of the 21st century. The laws governing social media require swift interpretation to keep pace with the ever-advancing criminal activity in this space,” said Mai Fernandez, executive director of the National Center for Victims of Crime. She said the justices had left victims in jeopardy.

“Threats play a central role in domestic abuse and is a core tactic that many abusers employ,” said Kim Gandy, president of the National Network to End Domestic Violence, adding that threats cause devastating harm “regardless of whether the abuser intended to threaten or only intended to vent or to make a joke.”

Writing about his estranged wife, Tara, Elonis had posted: “There’s one way to love you but a thousand ways to kill you. I’m not going to rest until your body is a mess, soaked in blood and dying from all the little cuts.”

At oral argument six months ago, justices seemed to agree there was a need for more than the “reasonable person” standard — which says that a reasonable person would consider a particular statement to be a threat — but there was no consensus on exactly what that standard should be. The limited ruling issued Monday and the length of time required to produce it indicated that no such agreement on a different standard had emerged.

Roberts said there is “no dispute” that the state-of-mind requirement is satisfied “if the defendant transmits a communication for the purpose of issuing a threat, or with knowledge that the communication will be viewed as a threat.”

But Alito, who agreed the case should be sent back to lower courts, said the decision left too many unanswered questions.

“The court refuses to explain what type of intent was necessary,” Alito complained. “Did the jury need to find that Elonis had the purpose of conveying a true threat? Was it enough if he knew that his words conveyed such a threat? Would recklessness suffice? . . . Attorneys and judges are left to guess.”

Paraphrasing the famous holding from Marbury v. Madison that it is the court’s prerogative to say what the law is, Alito said the court was announcing, “It is emphatically the prerogative of this court to say only what the law is not.”

The justices were considering a federal law that makes it a crime to communicate “any threat to injure the person of another.”

Prosecutors said there was no doubt Elonis was doing that on his Facebook feed during a two-month period in 2010. His wife had left with their two children, and Elonis, then 27 and working at an Allentown amusement park, grew increasingly despondent and angry.

He was fired and responded with a post about being a nuclear bomb about to explode. He pondered making a name for himself by shooting up an elementary school.

That brought a visit from an FBI agent, and the prolific Elonis later posted a fantasy about slitting the agent’s throat and turning her into a “ghost.”

Elonis was convicted after a judge told jurors that the government needed to prove only that Elonis made the statements and that a reasonable person would foresee that the words would be interpreted as “a serious expression of an intention to inflict bodily injury or take the life of an individual.”

Elonis served three years of a 44-month sentence before being released from prison.

The Philadelphia-based U.S. Court of Appeals for the 3rd Circuit upheld the conviction, saying Elonis’s subjective intent in writing his postings did not matter.

The Morning Call reported in April that Elonis had been arrested by police in Freemansburg, Pa., on charges of hitting his girlfriend’s mother with a pot.

The cases is Elonis v. U.S.