ALBUQUERQUE, N.M. >> A right-wing militia group operating in southern New Mexico has begun stopping groups of migrant families and detaining them at gunpoint before handing them over to Border Patrol agents, raising tension over the tactics of armed vigilantes along the border between the United States and Mexico.

The group, which calls itself the United Constitutional Patriots, filmed several of its actions in recent days, including the detention this week of about 200 migrants who had recently crossed the border near Sunland Park, New Mexico, with the intention of seeking asylum. They uploaded videos to social media of migrant families, blinking in the darkness in the glare of what appeared to be the militia’s spotlights.

The American Civil Liberties Union denounced the militia’s actions in a letter Thursday that asked New Mexico’s governor and attorney general to investigate the group.

In a statement, Hector Balderas, New Mexico’s attorney general, said: “These individuals should not attempt to exercise authority reserved for law enforcement.”

Jim Benvie, a spokesman for the United Constitutional Patriots, declined to specify how many of its members were in Sunland Park, a city in New Mexico about 9 miles west of El Paso, Texas. He said the group included people with military or law enforcement experience.

Gov. Michelle Lujan Grisham of New Mexico said in a statement it was “completely unacceptable” that migrant families “might be menaced or threatened in any way, shape or form when they arrive at our border.”

The video of this week’s episode in the New Mexico desert shows Border Patrol agents arriving on the scene at some point after members of the militia had come into contact with the migrants.

Before the arrival of the federal agents, 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.

Carlos A. Diaz, a spokesman for Customs and Border Protection declined to discuss the episode or the United Constitutional Patriots, but said the agency “does not endorse private groups or organizations taking enforcement matters into their own hands.”