supermodel Maggie Rizer's dog KILLED by united airlines

SUPERMODEL Maggie Rizer had done everything right, she thought.

The model had booked reservations for her two golden retrievers – 7-year-old Albert and 2-year-old Beatrice – on her flight from New York City to San Francisco through United Airlines's PetSafe program. But when Rizer & family landed in San Francisco on Sept. 3, they found out one of their family members had been lost along the way: Bea had died.

"We're completely devastated," Rizer tells PEOPLE. "[The airline] didn't even have an excuse."

The golden retriever's passing was inexplicable. Bea was young and had undergone a requisite physical examination – and was found to be in perfect health – just days before the flight. Golden retrievers don't usually experience problems during air travel.

In a passionate blog post on Rizer's website, Bea Makes Three, she paints a picture of United employees reluctant to admit wrongdoing or share any information about the company's internal investigation.

According to Rizer, her veterinarian performed a full necropsy and determined the cause of death to be heatstroke, "the absolute worst thing I could have imagined happening to her," Rizer says. "She died 30 feet below us, alone and scared. Nobody was there to help her because someone made a mistake."

In a statement, United tells PEOPLE that, "We understand that the loss of a beloved pet is difficult and express our condolences to Ms. Rizer and her family for their loss. After careful review, we found there were no mechanical operational issues with Bea's flight and also determined she was in a temperature-controlled environment for her entire journey. We would like [to] finalize the review but are unable until we receive a copy of the necropsy."

Rizer considered filing a lawsuit but hopes that speaking out about her experience will help inform people about the risks of traveling with a pet in cargo.

"I don't think dogs should be treated like bags," she says. "They're living, breathing creatures and parts of people's families." She adds that she will never fly with her dogs again.

"I'll miss her loving soul," Rizer says of Bea, who used to sit by little Zander and gently lick his face. "She was one of a kind. I've never had a dog like her. You can't replace a dog, especially one like that."