A Brooklyn Park man's incendiary Twitter posts advocating violence against police officers in retaliation for the deaths of unarmed black men in Missouri and New York have brought a national debate over the limits of free speech on social media uncomfortably close to home.

In a series of since-deleted tweets, the man talked about shooting police officers and decapitating them "like ISIS," authorities said. A later post reportedly read: "Opening fire on the police as well because they want to take me into custody for new charges."

The man, who has not been charged, later tweeted that his Twitter account had been hacked and that he harbored no animosity toward the police.

It remains unclear whether he was capable of carrying out the threats, but local authorities, already on edge over attacks on police elsewhere in the country, weren't taking any chances. After receiving phone calls and tweets from citizens alarmed by his posts, they launched an investigation into the man's online activities.

"It really comes down to a little bit of discretion. In this case, on the heels of two [New York] officers dying, then it certainly was our intention to look at it further," said Brooklyn Park Deputy Police Chief Mark Bruley.

Authorities did not charge the man because "he did not make a threat toward an individual," Bruley said.

Still, his department, like others across the country, has increasingly used social media as a crime-fighting tool.

Observers say the Brooklyn Park case bears an eerie resemblance to that of Ismaaiyl Brinsley, the Georgia man who killed two New York City officers, leaving behind a trail of electronic breadcrumbs.

Brinsley frequently posted anti-police and anti-government messages on social media, expressing anger over recent grand jury decisions not to indict white police officers in the deaths of unarmed black men in Missouri and New York. One post on Instagram, which appeared on the day of the killings, featured a pistol and bloodstained pants with the caption "I'm putting Wings on Pigs Today." Hours later, police say, he shot officers Wenjian Liu and Rafael Ramos as they sat in their squad car before taking his own life.

In a memo to police officers last week, Minneapolis Police Chief Janeé Harteau noted that the "past few weeks have been challenging for us all but especially for those of you on the front line."

She added: "Do not let the public debate and politics surrounding incidents across the country impact your focus on our task at hand and your successes in improving public safety in the city of Minneapolis."

In response to backlash from some who blamed protesters' criticism of police for motivating Brinsley, Minneapolis police union leader John Delmonico wrote on the union's website, "It is too easy and convenient to cast blame. The social media posts before the murders are certainly disturbing, but no one knows the extent to which the assassin's actions were influenced by mental health, protesters, elected officials or the media."

Rants vs. threats

Chris Uggen, a sociology professor at the University of Minnesota who studies criminal justice issues, said that both the New York and Brooklyn Park cases focused new attention on the scope of First Amendment protections for violence-laced messages and images posted online.

Most language found on social media, no matter how vitriolic and offensive, is protected by the First Amendment, Uggen said.

The problem for police, he said, is differentiating between run-of-the-mill social media rants and more specific, credible threats of violence.

"Indeed, we are seeing greater attention by law enforcement and prosecutors to this kind of speech, but the volume of material makes it very difficult to sort through what was the intent, what was the meaning of particular statements," Uggen said.

The U.S. Supreme Court recently took up the issue in a case involving a Pennsylvania man who was sentenced to nearly four years in federal prison for posting online rants against his co-workers, law enforcement agents and estranged wife.

What Uggen called "one of the clumsier attempts" to try to prosecute online posters has been over the use of hip-hop lyrics, "where people are expressing themselves in an artistic way and having that used against them in terms of gang activity."

He quickly added: "That's different, clearly, than the case in New York, where people are posting clearly about an intent to commit a crime."

Law enforcement agencies are wary of discussing their use of social media as an investigatory tool.

But investigators are turning more and more to social networking sites, particularly while tracking gang members, who they say use sites like Facebook and Twitter to coordinate criminal enterprises and recruit new members.

Uggen said the increased monitoring of social media by law enforcement has "not reached the point where people are feeling restricted or constrained."

"My sense is we'll become more vigilant in terms of what we say online and who will have access to it," he said. "At the same time, you'll have people who commit crimes who will leave a real trail of smoking-gun evidence."

Libor Jany • 612-673-4064 Twitter:@StribJany