Employees later found the dog in the freezer used to store dead animals, according to the veterinary board documents.

A jury convicted Dr. Koller of animal cruelty in that case, and he was sentenced to 100 days in jail.

The veterinary board in California was looking at a range of other concerns. In October 1974, regulators said, Dr. Koller pounded the head of a Lhasa apso dog named Tammy “until both eyes were hemorrhaged,” according to documents. In 1975, officials found evidence that he beat a dog named Coco for several minutes when the dog would not stop barking; bit and pounded a dog being prepared for surgery; used a slip lead to suspend a dachshund by the neck for at least 30 seconds; and violently slammed a cat on the sides and floor of its cage.

In 1979, the board revoked his license. Five years later, it reinstated it.

Dr. Koller expanded his work to Oregon in the 1980s and franchised a series of Companion Pet Clinics, offering lower-cost services than competitors.

Gretchen Kaehler, who briefly worked with him during that period, said the focus on a high-volume, low-cost model called for cost-saving measures like reusing needles — with sterilization — to the point that they would be difficult to insert into the animal.

Ms. Kaehler said she never saw Dr. Koller abuse animals, noting that he had a dog of his own, and that she recalled him being friendly and smart. But he was so focused on the business, Ms. Kaehler said, that he fired her for spending too much time comforting the animals.

“He wasn’t going to spend a lot of time petting them,” Ms. Kaehler said. “He didn’t have that nurturing side. It was very much just business.”