A teenager posts a Twitter message saying he's going to blow up a school. A husband grumbles that he's looking for a hit man to kill his wife. A wannabe jihadist says in an Internet chat room that he is ready to become a martyr.

Are any of them serious? Or is it all bluster?

Separating real threats from idle talk is a workaday task for law enforcement. It is rarely easy, but it has taken on extreme complexity in the lurid case of Gilberto Valle, a New York City police officer charged with plotting to kidnap, cook and eat women he knew.

At a conspiracy trial now in its second week, a jury has heard how Valle was part of an international community of fetishists who got their kicks trading wild fantasies online about violent acts against women.

By all accounts, he was into some sick stuff. After fighting crime at his day job, Valle spent his free time logging in to websites like Dark Fetish, where users posted accounts of rape, necrophilia and women being strangled and burned at the stake.

The site carried a disclaimer: "This place is about fantasies only." But prosecutors claim Valle took steps to get into closer contact with some of the women he wrote about, including using a police department database to look up their personal information, emailing and texting them more often and meeting with at least one of them.

Jurors in what the tabloids have dubbed the "Cannibal Cop" case will have their hands full when they begin deliberations, possibly as early as Thursday.

Valle's lawyer has argued that it was all clearly fiction. The plans Valle gruesomely described were never carried out. He never purchased the torture implements he described in emails with his fetishist pen pals. He never met the men accused of being his co-conspirators. The women he wrote about learned of the plans only after his arrest, with the exception of his wife, who discovered her husband's pastime after installing spyware on his computer.

As strange as the case is, experts said it touches on a common challenge in law enforcement: deciphering intent without running afoul of the First Amendment right to free speech.

"Simply thinking bad thoughts is not a crime anywhere," said David Raskin, a former assistant U.S. attorney who prosecuted terrorism conspiracies.

Nor is spouting off about violence on the Internet. So when terrorist sympathizers go online and talk about wanting to blow up buildings, the FBI will often send in an operative to tease out how far they are really willing to go.

"Obviously, they are very, very different types of offenses," Raskin said. "But it's the same challenge from the law enforcement perspective, which is, 'How do we get inside the guy's head and figure out if he will act on the things he is saying?'"

Last year, the FBI ran that type of test on Quazi Mohammad Rezwanul Ahsan Nafis, a college student from Bangladesh who fell under scrutiny after he began using social media to seek support for a terror attack.

The sting ended with the 21-year-old sitting in a van in front of the Federal Reserve Bank of New York, trying to detonate what he believed was a bomb. It was a fake, supplied by undercover agents. He pleaded guilty in February.

The FBI agents who investigated Valle never tried putting together a similar sting. The case against him is based mostly on his email exchanges.

Valle's attorney, Julia Gatto, opened her defense by arguing that the officer was being prosecuted purely for private speech.

"We don't convict people for sharing their thoughts, no matter how scary and disgusting they may be," she told the jury. "If we did, our prisons would be full with the directors of horror movies, with the producers of violent video games, with famous authors like Stephen King. We don't convict people because we don't like their thoughts. And you, ladies and gentlemen, are not deputy agents of the thought police."

A similar argument prevailed in the case of Jake Baker, a University of Michigan student who posted violent short stories about a female classmate in an Internet forum in 1994 and later exchanged emails with a man in which they discussed carrying out murder.

A federal appeals court ruled that the emails weren't "true threats," and the men were merely trying to "foster a friendship based on shared sexual fantasies."

If a jury reaches the same conclusion about Valle, it should acquit, said Andy Sellars, a staff lawyer for the Digital Medial Law Project at Harvard University's Berkman Center for Internet and Society.

"If this forum was nothing more than the sharing of repugnant stories, then we shouldn't hold them responsible," he said. "One of the things we protect in free speech is giving a person the opportunity to vent in order to relieve whatever disturbing thoughts they have on their mind."

Criminal law, he said, has historically given the benefit of the doubt to people who contemplate a crime but back away.

The catch in Valle's case is that he and the men he corresponded with talked in great detail about the twisted things they wanted to do. Maybe that was just to make the fantasy seem more real and feel more exciting. But maybe not.

The Russian entrepreneur who created Dark Fetish said in a video deposition taken in February that he himself had trouble discerning fact from fiction in some postings on his site, and had kicked users off for discussing things that sounded like real crimes.

"Let's say that it seemed not to be fantasy anymore," Sergey Merenkov said. "It could have led to something bad, yes."

Raskin, the former federal prosecutor, said that in arresting Valle, authorities did the safe thing. What if he really did kill someone?

"Just think of the story you would write if you found out that people knew about this guy before he did it. ... Half of law enforcement in New York City would get fired because of that story," Raskin said. "That is constantly on the mind of the people who are in charge of investigating these things. What if this is the guy who really isn't a wacko?"