A car was left running in the garage, and police are currently investigating the matter to determine if the car was left running on purpose as part of a murder-suicide plot.
Authorities were called to the property around 11:20 p.m. on Monday and found that it was filled with carbon monoxide.
When paramedics arrived, they tried to revive the family members, but it was too late.
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They are not currently looking for a suspect.
Victims included Gary Reitnauer, 59, Michelle, 59, and their adopted daughters Yun Jiao 'Kimberly' Cao, 16, and Jamie Hongfan Reitnauer, 10.
They plan to perform autopsies to determine the exact cause of death, though it is likely that it is from the carbon monoxide.
The police found Gary Reitnauer's body inside a car in the garage when they arrived at the house after receiving a call to check up on the family.
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Michelle and one child were found in the garage and the other girl was found inside the house.
Gary's nephew, Rob Zere, said, "The family was trying to save each other, and it didn't work out."
The school the girls attended is making arrangements for counselors to come in and help the staff and students cope.
The family was described as loving, and Gary always appeared happy.
They had lived in the house for 15 years and the children's grandmother lived on the property in a trailer.