From New York City to Kansas City, Mo., protesters and police authorities alike have acknowledged the seemingly strange phenomenon.

In almost no time, social media users monitoring the protests quickly floated theories about the arrival of the bricks.

Were they were strategically placed to incite riot? Conservative critics have openly wondered if the liberal militant group Antifa was behind the stashes, using them to encourage violence with agent provocateurs.

You can view clips below of social media account Breaking911 documenting bricks found in cities all across the nation:

No one is certain where the “mystery bricks” have come from — of it’s even a real trend.

President Donald Trump, Attorney General William Barr, and others have suggested the left-wing extremist group Antifa is to blame. Short for “anti-fascists,” Antifa has become an umbrella term for far-left-leaning militant groups.

Barr on Sunday said the FBI would use its regional joint terrorism task forces to “identify criminal organizers,” and Trump threatened again to name Antifa a terrorist organization.

Some, like Pablo Tirrante, had blamed Russian interference for the “pop up” bricks.

” … they stole the CIA playbook from the 70s and 80s… ” he wrote on Twitter.

U.S. officials sought to determine Sunday whether extremist groups had infiltrated police brutality protests across the country and deliberately tipped peaceful demonstrations toward violence — and if foreign adversaries were behind a burgeoning disinformation campaign on social media.

As demonstrations spread from Minneapolis to the White House, New York City and overseas, federal law enforcement officials insisted far-left groups were stoking violence. Meanwhile, experts who track extremist groups also reported seeing evidence of the far-right at work.

Investigators were also tracking online interference and looking into whether foreign agents were behind the effort. Officials have seen a surge of social media accounts with fewer than 200 followers created in the last month, a textbook sign of a disinformation effort.

The accounts have posted graphic images of the protests, material on police brutality, and material on the coronavirus pandemic that appeared designed to inflame tensions across the political divide, according to three administration officials. The officials spoke on condition of anonymity to discuss investigations.

The investigations are an attempt to identify the network of forces behind some of the most widespread outbreak of civil unrest in the U.S. in decades. Protests erupted in dozens of cities in recent days, triggered by the death of George Floyd, a black man who died after he was pinned at the neck by a white Minneapolis police officer.

Pandemic-weary Americans were already angry — about COVID-19 deaths, lockdown orders, and tens of millions of people out of work.

But there are signs of people with other disparate motives, including anarchist graffiti, arrests of some out-of-state protesters, and images circulating in extremist groups that suggest the involvement of outside groups.

Minnesota Gov. Tim Walz said Sunday that state authorities were hit with a cyberattack as law enforcement prepared to diffuse protests in Minneapolis and St. Paul, the epicenter of the unrest. He described it as a “very sophisticated denial of service attack on all computers.”

The Justice Department is also deploying members of the U.S. Marshals Service and agents from the Drug Enforcement Administration on Sunday to supplement National Guard troops outside the White House, a senior department official said. The official was not authorized to discuss the matter publicly and spoke on condition of anonymity.

The addition of the federal agents, who will have armored vehicles, came as AG Barr warned that prosecutors could seek to use terrorism statutes against “violent radical agitators” who attempt to hijack protests to cause destruction.

An Antifa-affiliated group disseminated a message in a Telegram channel on Saturday that encouraged people to consider Minnesota National Guard troops “easy targets,” two Defense Department officials said. The message encouraged activists to steal “kit,” meaning the weapons and body armor used by the soldiers. The officials were not authorized to discuss the matter publicly and spoke on condition of anonymity.

As a result, soldiers with the Minnesota National Guard were armed during their mission at protests across the state Sunday, the officials said. The soldiers are sometimes armed but had not been since they moved into parts of the state that had been besieged by riots in the last few days. The troops do not have the authority to make arrests, and are there to act mostly as extra security for police.

Others have seen evidence of right-wing extremists. J.J. MacNab, a fellow at George Washington University’s Program on Extremism, has been monitoring chatter about the protests among anti-government extremists on social media platforms. She has access to dozens of private Facebook groups for followers of the loosely organized “Boogaloo” movement, which uses an ’80s movie sequel as a code word for a second civil war.

“They want to co-opt them in order to start their war. They see themselves as being on the side of protesters and that the protesters themselves are useful in causing anarchy,” MacNab said.

She also sees signs that the Three Percenters militia movement appears to be taking an interest.

Trump was expected in the coming days to draw distinctions between the legitimate anger of peaceful protesters and the unacceptable actions of violent agitators, said a White House official who was not authorized to discuss the plans ahead of time and spoke on condition of anonymity.

America’s racial fault lines are a perfect opportunity for foreign adversaries looking to sow discord and portray the U.S. in a negative light, according to James Ludes, director of the Pell Center for International Relations and Public Policy at Salve Regina University in Rhode Island.

“This is a real issue and Americans are legitimately upset about it,” said Ludes, who studies foreign disinformation tactics. “That’s one of the hallmarks of these campaigns. You don’t create new issues, you exploit existing issues.”

There’s a history of this. In 2016, another black man, Philando Castile, was killed by police in a Minneapolis suburb, his death livestreamed on Facebook. Russians used a fake Black Lives Matter page to confuse and stoke anger among the protesters. There were nearly 700,000 followers, but it’s not clear how many were real.

One debunked example from this week: That Atlanta had deployed a “child militia.”

Floyd was accused of trying to pass a counterfit bill at a grocery store after he was laid off in the pandemic. Disturbing video showed him prone on the street, while a white police officer pressed his knee into Floyd’s neck even as he cried he couldn’t breathe. He later died. The officers have been fired; Derek Chauvin, the officer who pinned Floyd in the video, was charged with murder.

At first there were peaceful demonstrations, but violence soon erupted. A Minneapolis police station was torched and protests took off around the country, growing increasingly tense. Video showed a police vehicle ramming into demonstrators in New York. Meanwhile, a van with four New York Police Department officers inside was hit with a Molotov cocktail and torched.

Hundreds have been arrested nationwide and cities braced for more protests. But booking information from the county jail in Minneapolis, for example, showed that out of 59 protest-related arrests, 47 people had a home address in Minnesota, with the majority coming from the Twin Cities.

Before protests began in New York City, organizers of anarchist groups began raising money for bail, recruited medical teams to deploy for violent interactions with police, and planned how to target high-end stores, said John Miller, the NYPD’s deputy commissioner of intelligence and counterterrorism.

Scouts on bicycles would also move ahead of the groups to report where the police would be and then direct small breakaway groups to areas where they could torch police cars or throw Molotov cocktails, Miller said.

The NYPD has arrested 786 people related to protests since May 28 and 1 in 7 of them were not from New York City, he said.

In Washington, where protesters raged outside the White House, most of the 17 people arrested were from the area. D.C. Mayor Muriel Bowser said the groups seemed, at the least, organized to destroy with tools to break windows and distribute materials.