ATLANTA » Police agencies across the U.S. are on edge and on guard after receiving threats and calls for violence against them on social media in the aftermath of the killings of two black men and the sniper attack that left five officers dead in Dallas. Some departments ordered officers to pair up or more generally said they were heightening security.

Authorities have said the Dallas gunman, who also wounded seven other officers and two civilians, wanted to “exterminate” whites in the aftermath of the killings of Philando Castile in Minnesota and Alton Sterling in Louisiana. And a man who killed one person and wounded three others — including an officer — in Tennessee apparently told investigators he was motivated by the recent killings of black men by police.

Since then, threats ranging from generic promises of violence to specific video postings have been made, only heightening fears of further attacks. In Dallas, authorities received an anonymous threat Saturday, prompting police to tighten up security. Officers swarmed the department’s headquarters, searching for a reported suspicious person in a garage before finally issuing an all-clear.

While some threats have been unspecific and not credible, other promises of violence have been more targeted. In Louisiana, a 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 flashes a handgun in the video and talks about the slayings of Castile and Sterling.

Police also say a Wisconsin man posted calls on social media for black men to gun down white officers, and a woman in Illinois is accused of threatening in an online video to shoot and kill any officer who pulled her over.

In Mississippi, Waveland Police Chief David Allen was responding to threats of gunfire attacks that were phoned in and announced on social media.

“No problems have yet materialized, thank heavens,” Waveland Police Chief David Allen said in an email Sunday night to The Associated Press. “While we haven’t made any arrests, we are running down numerous leads given to us by the public.

“We been supplementing our shifts with more people and going to calls in groups.

“The outpouring of offers for help and shows of support from the public has really energized my entire department. At least in my area, police-public relations are strong and healthy.”

Mawuli Davis, an African-American attorney and activist in Atlanta, said what’s happening is a continuation of events in recent years because there has been no serious dialogue over issues of race and police encounters with black people.

Davis and his associates insist on peaceful protests as a means to an end, and indeed, most protests across the U.S. have gone on without a hint of violence. But until that serious discussion happens, he 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.

Tensions between police and African-Americans have been on the rise in recent years amid the high-profile deaths of several black men at the hands of law enforcement. The deaths have fomented unrest from Ferguson, Missouri, to Baltimore and heightened calls for greater accountability of police, particularly in the urban, majority-black neighborhoods they patrol.

While race has not necessarily been a factor in every case, the deaths have become a rallying cry for groups such as Black Lives Matter who are calling for solutions to problems that plague African-American communities, from poor educational opportunities to joblessness to high incarceration rates.

Organizations that monitor hate groups condemned the Dallas attack, with the Southern Poverty Law Center calling it “an act of domestic terrorism.” The gunman, Micah Johnson, followed black militant groups on social media.

Ryan Lenz, online editor and senior writer at the SPLC, told The Associated Press that the number of black separatist groups nearly doubled in 2015, mirroring a similar increase among white hate groups that has come against the backdrop of police killings that make frequent headlines.

While some committing violence may be influenced by hate groups, many who become radicalized do so without direct ties to the groups. Instead, they surf the web and allow their anger to grow in private at home, 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.”