(CNN) - A militia group near the US-Mexico border detained hundreds of people this week, New Mexico's attorney general told CNN.
"My office has been informed that this week, an armed group has detained nearly 300 people near Sunland Park, New Mexico," Attorney General Hector Balderas said in a written statement. "These individuals should not attempt to exercise authority reserved for law enforcement."
Videos posted online purportedly showing migrants held by the United Constitutional Patriots group and handed over to the US Border Patrol drew swift condemnation from the American Civil Liberties Union of New Mexico.
"We cannot allow racist and armed vigilantes to kidnap and detain people seeking asylum," the ACLU said in a letter to state authorities denouncing the actions and asking the government to step in. "We urge you to immediately investigate this atrocious and unlawful conduct."
CNN has reached out to the United Constitutional Patriots for comment.
The New York Times reported that a spokesman for the militia group said their actions were legal, "comparing the detention of the migrants to 'a verbal citizen's arrest.'"
Videos show armed men in masks, fatigues
Various private militia groups -- often espousing anti-immigrant views -- have patrolled the border for years.
But it's rare to see video of an armed group detaining migrants on the US side of the border.
Authorities say they're looking into videos posted on the United Constitutional Patriots New Mexico Border Ops Facebook page.
CNN has reached out to the group and the individuals who posted the videos for comment but has not confirmed details about who shot the videos, or when and where they were recorded.
The videos purport to show members of the group detaining migrants, including families with children, who've just crossed the border.
They show people often in full military fatigues, with handguns strapped to their sides, wearing gloves and black face masks. Armed men order migrants to stop, force them to sit on the ground and then apparently call Border Patrol to pick them up. At least two videos posted on the group's Facebook page depict a man in fatigues verbally identifying himself as "Border Patrol" as he stops a group of migrants.
CBP says it doesn't condone civilians interfering in law enforcement matters
Asked about the United Constitutional Patriots and the videos the group has posted online, a US Customs and Border Protection spokesman declined to comment on the group itself or the social media posts, but said the agency "does not endorse or condone private groups or organizations taking enforcement measures into their own hands."
"Interference by civilians in law enforcement matters could have public safety and legal consequences for all parties involved," the spokesman said in a written statement, referring those who suspect illegal activity to call 911 or contact the agency directly. "Border Security operations are complex and require highly trained professionals with adequate resources to protect the country."
And a CBP spokesman said the agency is looking into videos posted in which a member of the group appears to claim he works for the Border Patrol.
A statement on the United Constitutional Patriots' Facebook page describes the group as "Americans that believe in the Constitution and the rights of every American that will stand up for there rights in unity and help keep America safe."
"We're just here to support the Border Patrol and show the public the reality of the border," spokesman Jim Benvie told The New York Times, noting that his group had been camped near El Paso, Texas, for the past two months and intends to stay until President Trump's planned border wall is built.
But Border Patrol officials are distancing themselves from the group.
"We have contact with them, but they do not work with us. They do not work alongside us," one Border Patrol official said.
When it comes to protecting the border and making arrests, that authority is "strictly for Border Patrol and Customs and Border Protection," the official said, saying he hasn't seen anyone from the militia group attempt to make an arrest.
"By law, they cannot conduct an arrest," the official said. "If they do, then somebody needs to do something about it."
Governor's office calls threatening migrants 'unacceptable'
A spokesman for New Mexico Gov. Michelle Lujan Grisham said state and local authorities are looking into the matter.
"They have absolutely not been authorized by our office or any other. We are actively working with the AG's Office, state police and local police to determine what has gone on and what can and will be done," said Tripp Stelnicki, a spokesman for the governor. "That migrant families might be menaced or threatened in any way, shape or form is completely unacceptable."
Peter Simonson, executive director for the ACLU of New Mexico, told CNN that while militia groups have patrolled near the border before, now such groups feel empowered by rhetoric they hear from the White House.
"We concede that these groups have a freedom to associate, to assemble, freedom to speech and our state gun laws do give them ability to carry weapons," Simonson said. But the ACLU's key concern, he said, is that private armed citizens are "taking it upon themselves to carry out justice and not allowing federal authorities to do their job."
"These people are armed, their intentions are misguided and they certainly don't have training, much less any authority, to be conducting arrests and long-term detentions of people coming across the border," Simonson said. "We are concerned this is such a potentially explosive situation, we are worried someone is going to get hurt."
CNN's Carma Hassan contributed to this report.