Douglas, AZ – A couple died after they were found unresponsive at a Douglas home where carbon monoxide levels surpassed the lethal limit, authorities said.

Around 5:40 p.m. on Tuesday the Cochise County Sheriff’s Office and the Douglas Fire Department responded to a home near Hawthorne Avenue and Primrose Street for reports of suspicious activity.

Neighbors said a child was trying to get into the home, and the neighbors also tried to get the residents to open the door without success, according to the Sheriff’s Office.

Deputies and firefighters forced their way into the home and found a man and woman, both unresponsive. The couple, both 47 years old, was pronounced dead at the scene.

The 10-year-old child who tried to enter the home is the son of one of the victims, Carol Capas, spokesperson for the Sheriff’s Office, said.

Authorities tested the carbon monoxide levels inside the home and determined it was twice the lethal limit, according to the Sheriff’s Office. Capas said the levels measured were about 500 ppm.

The maximum carbon monoxide level that is considered safe indoors over eight hours is 9 ppm, and 200 ppm or more causes physical symptoms and is fatal in hours, Capas said.

“An outside heating unit was being used inside of the residence without proper ventilation likely resulting in the excessive build-up of carbon monoxide,” the Sheriff’s Office said.

The Pima County Medical Examiners Office will do the couple’s autopsies. The couple has not yet been identified.