A video has emerged of armed right-wing militia members stopping over 300 migrants as they cross the Mexico border into the US state of New Mexico.
The group calls itself the United Constitutional Patriots, and are seen in the Facebook clip standing over migrants sitting on the desert ground.
The group's actions have been condemned by civil liberties groups and New Mexico state officials.
The militia supports President Donald Trump's plan for a border wall.
The incident comes amid a spike in border crossings, despite White House efforts to stem the influx.
What does the video show?
The video, posted to militia member Jim Benvie's Facebook page, shows a large group of migrants who were said to have just crossed the border near Sunland Park, New Mexico, being detained by the armed vigilantes on 16 April.
The migrant group, which contains many women and children, are seen sitting and squatting in the darkness and squinting up into the militia's spotlights.
Posted by Debbie Collins Farnsworth on Monday, April 15, 2019
Before Border Patrol agents arrive to take custody of the migrants, a woman narrating the video tells a man who appears to be a militia member "don't aim the gun" in the direction of the families.
Mr Benvie, a spokesman for the United Constitutional Patriots, told the New York Times the group has been camping in the region for the past two months and plans to stay until Mr Trump succeeds in building his controversial border wall.
"If these people follow our verbal commands, we hold them until Border Patrol comes," Mr Benvie said, describing the interaction as a "citizen's arrest".
"Border Patrol has never asked us to stand down," said Mr Benvie, who is visiting the region from his home state of Minnesota.
What's the reaction?
New Mexico Governor Michelle Lujan Grisham said in a statement that the vigilantes had no legal right to stop anyone inside the US.
"If migrant families are made to feel threatened, that's completely unacceptable," she said.
"And it should go without saying that regular citizens have no authority to arrest or detain anyone."
New Mexico Attorney General Hector Balderas warned in a statement that "these individuals should not attempt to exercise authority reserved for law enforcement".
The American Civil Liberties Union described the group as an "armed fascist military organisation" in a letter to the governor and attorney general.
"The Trump administration's vile racism has emboldened white nationalists and fascists to flagrantly violate the law," they say.
"This has no place in our state: we cannot allow racist and armed vigilantes to kidnap and detain people seeking asylum. We urge you to immediately investigate this atrocious and unlawful conduct."
US Customs and Border Protection has previously said they are opposed to civilians patrolling the border in search of illegal crossers.