A Siberian Husky survived but had kidney failure and might be euthanized, veterinarian Dr. Laura Ivan told the St. Louis Post-Dispatch.
"I've never seen such a horrific act in my lifetime," Ivan said.
She called the deaths of the dogs -- including three golden retrievers, a Dalmatian, a different Siberian Husky, a Malamute and the top-ranked Akita -- a "horrible, tragic accident."
Mary Wild, 24, a paid handler, was not charged in the deaths, although a sheriff's department report will be turned over to the Jefferson County prosecutor, police said.
Wild told police she left the dogs in a cargo van in the driveway around 1 a.m. CDT Monday and went to bed after returning from a dog show in Iowa, police said.
She explained she put six electric fans in the van to keep the dogs cool. She said she also left a van door open and the van's windows partly open, Capt. Ralph Brown of the sheriff's office told the newspaper.
She told police she checked on the dogs around 4 a.m. and they appeared fine, Brown said.
But about 6:30 a.m., all eight dogs were in distress, Brown said. Five were breathing, but not responsive. The other three could raise their heads.
The National Weather Service said the outside temperature at 1 a.m. Monday was 83 degrees and 80 degrees at 6 a.m.
Investigators were trying to determine how hot the inside of the van could have gotten.
Wild did not return phone and e-mail messages from the Post-Dispatch requesting comment.