DOUGLAS, Arizona (KVOA) — A couple found dead in a home near Douglas on Tuesday likely died of carbon monoxide poisoning, officials say.

At approximately 5:40 p.m., authorities responded to reports of suspicious activity at a residence at 3000 Hawthorne Avenue in Bay Acre.

in a facebook post, The Cochise County Sheriff’s Office said a neighbor advised a child was trying to break into their home before first responders arrived on the scene.

They said a 47-year-old man and woman were pronounced dead at their residence.

First responders tested carbon monoxide levels in their homes. They were determined to be twice the lethal limit.

“Excessive carbon monoxide build-up may have occurred due to the use of outdoor heating units in dwellings without adequate ventilation,” CCSO said.

The couple were taken to the Pima County coroner’s office to complete an autopsy, CCSO said.

Details have not been released, but the incident is under investigation.