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.

Authorities are now investigating whether the Dallas gunman was directed by groups such as the African American Defense League or the New Black Panther Party or merely emboldened by them.

“I think it’s safe to say we’ll leave no stone unturned,” Dallas Deputy 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 the hate speech the groups espouse on social media?

Ryan Lenz, online editor and senior writer at the Southern Poverty Law Center, said 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.

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.”

In addition, white supremacist groups have made a resurgence in the years since President Barack Obama was elected as the first black president.

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.

(AP)