Authorities announced the boy was in custody at a news conference in Burlington but gave few details and did not take any questions.
The boy's name was not released because he is a juvenile.
"The case has been referred to the district attorney's office for the filing of formal charges," said Steve Johnson, the assistant director of the Colorado Bureau of Investigation.
Police found the bodies of Charles Long, 50, and his wife Marilyn, 51, in their Burlington, Colo., home Tuesday night after receiving a 911 call from the 12-year-old boy, ABC affiliate KMGH-TV reported.
A 5-year old sister and 9-year old brother also were found critically wounded and were airlifted to a Denver hospital, KMGH reported.
The 12-year-old reportedly was not hurt.
"There are no other suspects at this time," Johnson said.
The boy told police that shots had been fired and three people were shot, according to Johnson.
The Longs were the parents to seven children, KMGH reported, but most of them were older and have moved out.
Neighbors told the station that the three children at home during the shooting were home schooled.
"It's shocking," neighbor Josh Schlichenmayer told KMGH. "They're good Christian people. They go to church. As far as I know, they had a happy family. You don't expect this to happen in a small town like this."