Members of the group, which calls itself the United Constitutional Patriots, filmed several of their actions in recent days, including the detention this week of a group of about 200 migrants who had recently crossed the border near Sunland Park, N.M., with the intention of seeking asylum. They uploaded videos to social media of exhausted looking 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 on Thursday that asked New Mexico’s governor and attorney general to investigate the group. The A.C.L.U. said the militia had no legal authority under New Mexico or federal law to detain or arrest migrants in the United States.

“We cannot allow racist and armed vigilantes to kidnap and detain people seeking asylum,” two lawyers for the A.C.L.U., María Martínez Sánchez and Kirsten Greer Love, said in the letter.