"These individuals should not attempt to exercise authority reserved for law enforcement," said New Mexico Attorney General Hector Balderas.

Paul Ratje / AFP / Getty Images Dozens of people who had just crossed the border are detained in Sunland Park, New Mexico, where members of the United Constitutional Patriots patrolled.

Militia groups rallying against undocumented immigrants have operated at the southern border for years, but state and federal officials issued warnings this week after one group began detaining hundreds of people crossing the border at gunpoint without legal authority. For months, the right-wing militia group known as United Constitutional Patriots has posted videos online from the border, recently showing its members stopping and detaining people in the New Mexico desert.

"Don't aim the gun," a woman narrating one such video posted by the group is heard saying moments before Border Patrol agents arrive to apprehend the detained immigrant families.

In a 41-minute video posted by the group this week, a large gathering of people, including several young children, are seen sitting in dirt as members of the militia encircle. Children are bundled up in sweaters, some wearing backpacks, while illuminated by flashlights. "This group was so huge we won't have an accurate count until the [Border Patrol] is finished processing them," the group said on its Facebook page. "This needs to stop!!!!! Build the wall." Some of the people in the video are heard coughing and asking for water. When asked why they entered the US, some said they were seeking asylum.

In another video, a member of the militia is heard yelling in Spanish, "Pistola, pistola" — Spanish for gun — at a group of people moments after they crossed the border at night. Moments later, one of the men is heard yelling in Spanish, "Policía, alto!" or "police stop!"

A member of the group then approaches a woman crouching down and covering a young child with her body, telling her, "No reason to be scared, c'mon." In another video posted by the group, a Border Patrol official is seen leaving five women and five children in the custody of the militia, assuring them another agent was on their way. While a handful of right-wing militia groups have operated at the border for some time, they have generally had limited contact with people crossing the border, instead acting as an extra set of eyes and ears for Border Patrol agents, alerting authorities of crossings. In recent local news reports, the United Constitutional Patriots expressed support for President Trump and his effort to build a border wall and said the group descended on the area "to document the crisis on the border." On Thursday, however, state officials publicly condemned the United Constitutional Patriots after the militia detained nearly 300 people near Sunland Park, New Mexico, at gunpoint. "These individuals should not attempt to exercise authority reserved for law enforcement," New Mexico Attorney General Hector Balderas said in a statement. United Constitutional Patriots is headquartered in Flora Vista, New Mexico, in the northern part of the state, according to the Southern Poverty Law Center. It's unclear how many members belong to the group.

Paul Ratje / AFP / Getty Images A member of the United Constitutional Patriots near the US–Mexico border in New Mexico.

Like several other militia groups, the organization issued a call to members last year as a large caravan of people, many seeking asylum, headed toward the US. The group has also looked to raise funds online, asking for supplies, gas money, and cash to maintain an ongoing presence at the border. One of the United Constitutional Patriots' fundraising efforts, however, was cut off when the crowdfunding site GoFundMe shut down the page for violating its terms of service, a spokesperson for the company told BuzzFeed News. The American Civil Liberties Union on Thursday asked New Mexico's governor and attorney general to investigate the militia. "Their actions undermine the legitimate efforts of our state’s law enforcement officials to keep New Mexico families safe and they erode community trust," attorneys for the civil rights organization said in a letter. "We cannot allow racist and armed vigilantes to kidnap and detain people seeking asylum." In a statement, US Customs and Border Protection said the agency does not endorse private groups or organizations taking enforcement matters into their own hands. "Interference by civilians in law enforcement matters could have public safety and legal consequences for all parties involved," a CBP official said. "Border Security operations are complex and require highly trained professionals with adequate resources to protect the country."