The 14-year-old boy was being held Tuesday night at the Quay County Juvenile Detention Center. The Associated Press is not identifying the teen because he is a juvenile.
A call came into to police before midnight Monday about shots fired at a home near Tucumcari. Officers arrived to find the bodies of 67-year-old Sue Day and her biological daughter, 49-year-old Sherry Folts.
The teen, who had been adopted by the Day family two years ago, was arrested and charged following an interview in which police say the boy indicated that he had stabbed Folts and shot Day.
"He has not given us a motive yet," State Police Capt. Ryan Suggs told Albuquerque television station KOB-TV. "But no doubt, this is an unfortunate event, and you can't make sense of it."
The Day family has taken in dozens of foster children over the last several years, according to the state Children, Youth and Families Department. The agency said it had complaints about the Days and their foster children but never found any evidence of abuse.
On Monday night, police said there was a 9-month-old girl inside the home along with a 16-year-old adopted boy and adoptive father Mike Day. Police say Mike Day and the older boy were able to restrain the 14-year-old before calling police.
CYFD spokesman Enrique Knell told television station KRQE that it was the older boy who saved the infant's life.
"He basically wrestled with his adopted brother, taking him to the ground, knocked out his two front teeth and stopped him from what was about to go on," Knell said.
State police said the investigation was ongoing.