"We can not allow racist and armed vigilantes to kidnap and detain people seeking asylum", lawyers for the American Civil Liberties Union said in a statement.
The FBI said it arrested 69-year-old Larry Mitchell Hopkins, of Flora Vista, in Sunland Park.
New Mexico Attorney General Hector Balderas warned in a statement that "these individuals should not attempt to exercise authority reserved for law enforcement".
A child who illegally crossed the border with her father near Yuma this week was discovered to have strep throat, Customs and Border Patrol officials say.
On April 16, the mayor of Yuma declared a state of emergency as a result of the number of migrant families being released from Border Patrol custody into the city.
A group of 360 were apprehended near the Ajo Station in Tucson after agents spotted multiple buses arriving near the border.
Female zoo keeper attacked by tiger at Topeka Zoo
Wiley went on to share that despite the "unfortunate incident", Sanjiv, who recently fathered 4 cubs, would not be euthanized. They say this type of attack has never happened at the zoo before, and they will use this incident in future safety protocol.
"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 for the past two months. "Border Patrol has never asked us to stand down". The ACLU cites videos posted on social media showing members of a militia group pursuing migrants in New Mexico's desert west of El Paso, Texas, and effectively kidnapping them against their will.
The UCP has previously presented itself as a group of "volunteers" aiding border patrol and supporting Trump.
Mexico said on Saturday it had "deep concern" about armed groups that intimidate and extort migrants on the border, shortly after the ACLU and Democratic senators called for a probe into such citizen efforts to block migrants from crossing.
U.S. Customs and Border Protection said in a statement early Friday it "does not endorse or condone private groups or organizations that take enforcement matters into their own hands".
"Our next focus is to try to get their story and making sure they feel safe to tell their story of being held at gunpoint", she told the Guardian, adding that the ACLU would explore legal options.