Dallas, TX – Authorities suspect carbon monoxide poisoning in the deaths of three people whose bodies have been found in a house in eastern Dallas.

A Dallas Fire-Rescue statement says the situation came to light about 7:30 p.m. Wednesday when a relative of the family inside came to the house but got no response at the front door. Concerned, he called 911 and arriving firefighters found the bodies inside, detected high levels of carbon monoxide and called in the hazardous materials team to the house about 3 miles (5 km) east of Fair Park.

Fire-Rescue spokesman Jason Evans says it’ll be up to the medical examiner to make the final determination of the death cause. Meanwhile, investigators are searching for the source of the fumes.