Firefighters said a Dallas family of three apparently died of carbon monoxide poisoning.
Dallas Fire-Rescue said a man and woman in their 40s and their teenage daughter were found dead in the back of their Pleasant Grove home just off Buckner Boulevard and Lake June Road on Monday.
Family members of the victims called authorities when they became concerned after not hearing from the family in more than 24 hours.
Dallas Fire-Rescue spokesman Jason Evans said firefighters took a carbon monoxide reading after forcing their way into the house.
"The meter read 690. When it reads 35 to 40, that's considered dangerous," he said.
Firefighters said the family was trying to heat their house with a 20-gallon propane heater meant for outdoor use, unaware that it produces carbon monoxide.
Investigators said they believe the family may died Sunday evening.
Luis Berea, a relative of the victims, said he and rest of his family were shocked by the deaths.
"To me, he was on of my favorite uncles," he said. "I grew up with him, my dad grew up with him. My cousin, his wife, they were good people, I don't know."
"Nobody expected this, especially this time of year," Berea said.