Police responding to a welfare check encountered strong chemical fumes coming from inside the house and a "cryptic" note on the door, the reports said.
After officers were able to enter the home, they found the bodies of a couple, both in their 30s, along with their four children. They were all inside the vehicle in what was believed to be a case of murder-suicide, San Antonio police Chief William McManus said at a news conference.
"Saying it's not pretty ... there's no words to describe that," he added.
The children's ages ranged from 11 months to 4 years old, the chief said. The family's two pet cats also were found dead in the SUV. The names of the family members were not immediately released.
"It's the whole picture. The adults, the children, the pets," McManus said.
Police were originally called to the home for a welfare check after the father, a military service member, failed to check in to work, which he had started doing from home, San Antonio's WOAI-TV reported.
Seven officers responded but smelled “a heavy, noxious fume coming from inside," police said. McManus later added that the odor was carbon monoxide.
“[It] kind of blew everybody back out the door," McManus said.
Police also found a "cryptic note" on the door written in military jargon that said bodies could be found inside. The note reportedly added that animals could be found in a freezer.
“On the door here was a note, very cryptic, some of it was military jargon, I’m told,” McManus said, according to the station. “The interpretation of it was, there were people, bodies or people, inside and do not enter.”
The officers who originally responded to the call received a medical check and were cleared, he added.
Police said a drone later sent inside the house found what was believed to be possible explosives. It forced the evacuation of more than 200 homes in the area around 5 p.m., according to The San Antonio Express-News.
"Police and Fire are evacuating the area in the 100 block of Red Willow for possible explosive materials. Please stay away from this area as first responders work this scene," the San Antonio Police Department wrote on Facebook.
CLICK HERE FOR THE FOX NEWS APP
No such explosives were found when police eventually entered and found the bodies, McManus said. He added it wasn't clear which military branch the man was with.
The family had reportedly moved back into the home in January after residing elsewhere.
The Associated Press contributed to this report