Tensions are mounting in the US city of Portland, where federal troops have been sent to end almost two months of anti-racism protests, some of them violent.
The protests began after the death of George Floyd in police custody in May.
There have been nightly rallies ever since, with clashes between protesters and police escalating in recent days.
President Donald Trump says he is trying to restore order. The approach has been criticised by local officials as an election-year ploy to try and paint his opponents as weak on law and order.
What has been going on in Portland?
Like other US cities, Portland, Oregon saw a wave of peaceful protests over Floyd's death in Minneapolis, with thousands taking to the streets to demand police reform and racial equality.
Recent protests, however, have been marred by violence, arson, damage to property, arrests and allegations of police brutality.
Last week, federal officers started cracking down on crowds gathering in the city against the wishes of state and local officials, drawing widespread criticism and legal challenges.
Federal officers in unmarked vehicles appeared to forcefully seize protesters from the streets and detain them without justification. They have also fired tear gas and less-lethal munitions into crowds of demonstrators.
These federal officers (?) just rushed up and arrested someone for no reason pic.twitter.com/xcFVuoMZmN— Matcha chai (@matcha_chai) July 15, 2020
Homeland Security Secretary Chad Wolf has called the protesters a "violent mob" and "anarchists".
But officials in the city have drawn a distinction between the peaceful majority and smaller groups of troublemakers.
Facing an election in November, President Trump has pledged to bring the unrest under control, blaming Democrats for allowing protests to descend into violence.
"We are trying to help Portland, not hurt it. Their leadership has, for months, lost control of the anarchists and agitators," Mr Trump tweeted on Sunday.
Who are the federal officers in Portland?
The role of federal troops sent to Portland is the subject of intense speculation at the moment. Numerous widely-shared videos show troops in camouflaged combat-style uniform arresting protesters, transporting them away in apparently unmarked vehicles.
Senior Democrat Nancy Pelosi said "unidentified Stormtroopers" were "kidnapping protesters". Another Democrat Ro Khanna described those involved as "secret federal agents".
So, who exactly are they?
They belong to a new federal force created last month in an executive order signed by President Trump which tasks them to protect historic monuments, memorials, statues, and federal facilities.
The personnel are drawn from a range of teams including the Customs and Border Protection (CBP) agency and the US Marshals Service. The Federal Protection Service, part of the Homeland Security and charged with protecting government buildings, also has officers in Portland and has detained protesters.
The US Marshals Service's Special Operations Group says it is "deployed in high-risk and sensitive law enforcement situations, national emergencies, civil disorder and natural disasters."
When asked about the arrest of a protester captured on video, the CBP said the individual was suspected of destroying federal property. They said agents had identified themselves and were wearing CBP insignia but their names were not displayed "due to recent doxing incidents against law enforcement personnel who serve and protect our country."
Some of those CBP agents in Portland photographed in camouflage are in the Border Patrol Tactical Unit, the agency's equivalent to a Swat team.
What reaction has there been?
Portland Mayor Ted Wheeler, a Democrat, said the "dozens if not hundreds of federal troops" were "sharply escalating the situation".
"Their presence here is actually leading to more violence and more vandalism," he told CNN on Sunday, renewing his call for the troops to leave the city.
Oregon Governor Kate Brown, also a Democrat, mirrored those comments, accusing President Trump of sending federal troops to the city for "political theatre".
The state of Oregon filed a lawsuit against the federal agencies involved last week, accusing them of unlawfully detaining protesters. Attorney General Ellen Rosenblum requested a restraining order to stop agents arresting people.
Given this legal challenge, the American Civil Liberties Union said the situation in Portland amounted to a "constitutional crisis".
But some activists have criticised the violence in the city, blaming it on a small minority of trouble-causers.
Civil rights advocate Ron Herndon told the Associated Press some were "taking advantage of the demonstrations for their own reasons that have nothing to do with social justice".
What happens next?
The state of Oregon is awaiting the outcome of its lawsuit against the federal government.
Meanwhile, the White House is said to be considering other ways it can use federal law enforcement agencies to quell civil unrest nationwide.
"You'll see something rolled out this week as we start to go in and make sure that the communities, whether it's Chicago or Portland, or Milwaukee, or someplace across the heartland of the country, we need to make sure our communities are safe," White House Chief of Staff Mark Meadows told Fox News on Sunday, without giving specifics.