In the hours after President Joe Biden’s inauguration this year, protesters marched once again through the streets of Portland, Oregon, sending a message that putting a Democrat in the White House would not resolve their problems with a system of policing and corporate wealth that they saw as fundamentally unfair.

“No cops, no prisons, total abolition,” they chanted. Some of the activists, dressed in the trademark uniform of solid black clothing and masks that often signals a readiness to make trouble without being readily identifiable, smashed windows at the local Democratic Party headquarters.

The event — like others that had consumed the city since the murder of George Floyd by a police officer in Minneapolis in 2020 — included a variety of anarchists, anti-fascists, communists and racial justice activists. But there were others mingling in the crowd that day: plainclothes agents from the Federal Bureau of Investigation.

The FBI set up extensive surveillance operations inside Portland’s protest movement, according to documents obtained by The New York Times and current and former federal officials, with agents standing shoulder to shoulder with activists, tailing vandalism suspects to guide the local police toward arrests and furtively videotaping inside one of the country’s most active domestic protest movements.

The breadth of FBI involvement in Portland and other cities where federal teams were deployed at street protests became a point of concern for some within the bureau and the Justice Department who worried that it could undermine the First Amendment right to wage protest against the government, according to two officials familiar with the discussions.

Some within the agencies worried that the teams could be compared to FBI surveillance transgressions of decades past, such as the COINTELPRO projects that sought to spy on and disrupt various activist groups in the 1950s and 1960s, according to the officials, one current and one former, who spoke on condition of anonymity because they were not authorized to discuss the debate.

There has been no evidence that the bureau used similar surveillance teams on right-wing demonstrators during the Jan. 6 riot at the U.S. Capitol, despite potential threats of violence against the heart of federal government — though the FBI did have an informant in the crowd that day. The bureau has at times used secretive tactics to disrupt right-wing violence, such as efforts that led to charges against men accused of conspiring to kidnap Michigan’s governor.

The FBI has broad latitude to conduct surveillance when agents suspect threats to national security or that federal crimes may be committed. But bureau guidelines warn that agents should not cross into actions that could have a chilling effect on legitimate protest, and should instead prioritize less-intrusive techniques.

In Portland, federal teams were initially dispatched in July 2020 to protect the city’s federal courthouse after protesters lit fires, smashed windows and lobbed fireworks at law enforcement personnel in the area. One demonstrator had attacked a federal officer with a hammer. But the FBI role quickly widened, persisting months after activists turned their attention away from the courthouse, with some targeting storefronts or local institutions whose protection would normally be up to the local police.

Both local and federal law enforcement officials have complained that lawful peaceful protests were hijacked in many cases by criminals.

But organizers of the protests and civil rights groups, after being told of The Times’ findings, said that surveillance agents recording and following protesters in the midst of a demonstration was a form of domestic spying.

“These are all insidious tactics that chill First Amendment expression and erode trust with local officials,” said Bobbin Singh, executive director of the Oregon Justice Resource Center, one of several civil rights organizations that objected to the mass arrests and violent crackdowns that followed the protests. He called the government’s operations an “alarming” misuse of resources.

Kieran L. Ramsey, the FBI’s special agent in charge of the Portland field office, said the office was committed to pursuing “violent instigators who exploit legitimate, peaceful protests and engage in violations of federal law.”

“At all times, our focus was on those planning or committing significant criminal activity or acts of violence,” Ramsey said in a statement.

Police officers made more than 1,000 arrests during the course of the protests, and more than 200 people ultimately faced criminal prosecution. More than 100 cases had to be dropped because there was not sufficient evidence.

In fast-moving street gatherings where people concealed their identities and demanded that cameras not be present, working invisibly inside the crowd may have given the authorities more opportunity to identify and apprehend those engaging in the most serious mayhem.

In one case, FBI agents in plain clothing were credited in court records with helping catch a man accused of throwing Molotov cocktails at law enforcement officers. He faced federal explosives charges in addition to state charges that included attempted murder.

The FBI teams continued their operations among Portland’s far-left activists for months at the end of 2020 and the start of 2021. While the FBI has also been investigating far-right groups, some lawmakers have blasted the bureau for failing to detect and blunt the Jan. 6 attack on the U.S. Capitol.

Renn Cannon, who was the Portland office’s special agent in charge during the demonstrations until he departed early this year, said that there were persistent protest-related crimes and tense political dynamics, leaving the bureau to try to address the crimes while also upholding First Amendment rights.

“I thought a lot about what is allowed under the Constitution,” Cannon said. “How do you do surveillance effectively, safely and legally? That was something we spent a lot of time on.”

Cannon declined to discuss specific operations or tactics but said he believed that his agents had crossed no lines while trying to make sure that laws were enforced.

In the middle of his reelection campaign, President Donald Trump vowed to “dominate” protesters who had taken to the streets in the wake of Floyd’s death, and he directed federal agencies to deploy personnel to protect federal property around the country. Outrage and even larger mass protests ensued in Portland after videos showed federal agents in tactical gear seizing people off the streets into unmarked vehicles and one agent beating a Navy veteran with a baton.

FBI officials heeded the call for action. David L. Bowdich, who was then the FBI’s second-in-command, had called the protests after Floyd’s murder “a national crisis” in a memo. He likened the situation to Sept. 11 and suggested that the bureau could make federal criminal cases against protesters by using the Hobbs Act — a law from the 1940s that was designed to crack down on racketeering in labor groups.

FBI Director Christopher Wray told lawmakers in September 2020 that the bureau was pursuing “quite a number of properly predicated domestic terrorism investigations into violent anarchist extremists, any number of whom self-identify with the antifa movement.”

The FBI is aggressively investigating people associated with violent far-right groups such as Atomwaffen and the Base, and prosecutors have already brought charges against dozens of members of the far-right Proud Boys and the Oath Keepers militia in connection with the attack on the Capitol. Federal agents are actively pursuing additional cases against those groups and further charges are likely to be filed.

Those investigations have sometimes involved confidential informants and surveillance. But no other evidence has emerged that FBI agents in recent years had blended into crowds engaged in political protests in the streets.

Later, after the overt federal crackdown in Portland ebbed and protest crowds waned, smaller groups of activists continued demonstrations that frequently included smashed windows and fires at buildings such as the headquarters of the Portland Police Association.

Agents from the FBI were still on the ground. In early November 2020, according to records reviewed by The Times, federal agents at one demonstration were “conducting surveillance in the crowd.” As a group marched near the Portland State University campus, some in the crowd shattered windows at a Starbucks.

An FBI special agent who reported being “in a plainclothes surveillance capacity” described witnessing one of the demonstrators break out a Starbucks window with a tire iron before placing the tire iron back inside his backpack, according to a written Portland Police Bureau summary of the federal agent’s account. In the report, the police officer wrote that he had been asked not to identify the federal agent’s name in documents.

The following week, according to an email between an FBI agent and a Portland police officer, FBI agents were again in the crowd conducting surveillance. One of the FBI agents captured a 30-minute video of the scene as he appeared to stand next to a crowd of demonstrators while others smashed windows at a Democratic Party building. The video shows the agent then joining the crowd as it marched down the street.

One of the agents later reported in records seeing an agent from the Department of Homeland Security also on the scene.

Sen. Ron Wyden, D-Ore., who has been scrutinizing the federal response to Portland, said that while federal officers have a right and a responsibility to protect federal property, there should be a high bar when it comes to agencies surveilling political gatherings.

“The Department of Justice needs to explain to me why it deployed those teams and provide a real record of their activities,” Wyden said. “What were they there for? Were they there primarily to chill peaceful protesters, or were they there to protect federal property?”

At the Inauguration Day demonstration in January, about 200 people gathered. “We are ungovernable,” one of their signs said.

Local and federal law enforcement records show that about half a dozen federal agents were there that day, with at least some of them doing what was described as surveillance in which they planned to follow protesters who engaged in property crimes or violence — even though the protest that day was starting on the east side of the city, far from the federal properties downtown. Agents singled out and tracked several people who had broken windows, trailing the individuals for several blocks until local law enforcement agents detained them.

Four agents testified before a local grand jury that was considering indictments against protesters who had been arrested. A person familiar with the proceedings said one of the agents testified that the federal officers had been wearing black apparel, a fact that suggests the agents were attempting to disguise themselves as protesters. FBI officials declined to discuss specific tactics or clothing used during their operations.

Prosecutors obtained indictments for six people on riot and criminal mischief charges.

Mike German, a former FBI special agent who specialized in domestic terrorism and covert operations and is now a fellow at the Brennan Center for Justice, said that such surveillance operations inherently run the risk of violating First Amendment rights. They should be used only when there is evidence that a serious crime may occur, he said, and they should be tailored to focus on obtaining the evidence needed to prosecute that crime.

“The FBI should focus its resources on groups engaged in deadly violence, not vandals,” he said.

Cannon, the former FBI supervisor in Portland, said the bureau was indeed worried about acts of violence directed at the police, the potential that the protests could escalate and the toll the demonstrations were taking on the Portland Police Bureau, whose officers were fatigued after months of near-nightly confrontations on the streets.

“This was a wave of protest-related crimes that had a severe impact on the community,” Cannon said. “There was a lot of pressure. It was a fraught situation.”