DALLAS (AP) — Police shootings of black men in Louisiana and Minnesota were followed by calls from black militant groups and others to seek vengeance against officers. Almost immediately, several officers were attacked, including the five slain by a sniper in Dallas.

Now authorities are investigating whether the Dallas gunman was directed by those groups or merely emboldened by them.

“I think it’s safe to say we’ll leave no stone unturned,” Dallas Deputy Police Chief Scott Walton said.

Police have been tight-lipped about exactly what they’re investigating and what they’ve uncovered so far. Although Micah Johnson was connected to several militant groups on social media, it’s unclear if he was merely a follower or a more active participant.

Similar questions have been raised by international terrorist organizations such as the Islamic State group: How is the network encouraging and directing attacks? Is it a coordinated effort or are the attacks simply a byproduct of hate speech espoused by the groups on social media?

The number of black separatist groups nearly doubled in 2015, mirroring a similar increase among white hate groups that has taken place as police killings make frequent headlines, said Ryan Lenz, online editor and senior writer at the Southern Poverty Law Center.

Still, many people who become radicalized do so without direct ties to any groups. Instead, they surf the web and grow their anger in private, Lenz said.

“In the last couple of years, we’ve seen this violence become an ever-present reality in our lives,” Lenz said. “We are in a polarized political climate right now where the ‘us-versus-them’ mentality has started to reign supreme.”

Johnson followed black militant groups on Facebook, including the African American Defense League, which posted a message that referenced the police shooting of Alton Sterling in Baton Rouge, Louisiana: “You and I know what we must do and I don’t mean marching, making a lot of noise, or attending conventions. We must ‘Rally The Troops!’ It is time to visit Louisiana and hold a barbeque.”

Other groups Johnson “liked” included the New Black Panther Party, the Nation of Islam and the Black Riders Liberation Party. The last two are described as hate groups by the law center, which monitors hate crimes and right-wing extremism.

Johnson’s Facebook photo showed him wearing a dashiki and raising his fist over the words “Black Power.” His cover shot carried the red, black and green Pan-African flag.

There’s no evidence such groups have directed violent events, but their rhetoric has served as inspiration, Lenz said.

Law enforcement agencies across the U.S. are on guard for threats after the police killings and the Dallas attack. Protesters view the police slayings as further evidence of the law enforcement abuse that has energized the Black Lives Matter movement, which was fueled by the 2014 killing of Michael Brown by an officer in Ferguson, Missouri.

Recent threats ranged from generic promises of violence to specific video posts. In Dallas, officers swarmed police department headquarters Saturday after a report of a suspicious person in a garage before finally issuing an all-clear.

A Louisiana man was accused of posting a video online showing him in his vehicle behind a police car, saying he wanted to shoot and kill an officer. Police say Kemonte Gilmore flashed a handgun in the video and talked about the slayings of Philando Castile in Minnesota and Alton Sterling in Louisiana.

In Wisconsin, a man posted calls on social media for black men to gun down white officers, and a woman in Illinois threatened in an online video to shoot and kill any officer who pulled her over, police said.

Mawuli Davis, an African-American attorney and activist in Atlanta, said the unrest continues because there has been no serious dialogue about issues of race and policing.

Davis and his associates insist on peaceful protests as a means to an end, and most protests across the U.S. have gone on without a hint of violence. But until that discussion happens, Davis said, he fears “we’re going to continue to see this kind of tragic incident” like the Dallas attack.

“From an activist perspective, you’re seeing a level of frustration and anger that very well may be at a tipping point,” he said.

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