(CNN) For pets, the skies of United Airlines lately have not been so friendly.

It's not just the French bulldog that died on a Houston-to-New York flight Monday after a United flight attendant told its owners to put the dog in an overhead bin.

Data from the US Department of Transportation show that three times as many animals died on United flights last year than on all the other US carriers put together. As you can see from this chart, 2017 wasn't an exception.

United has had the highest rate of pet deaths of any US airline for the past three years

In fairness, the airline in recent years has also been the largest transporter of animals -- mostly dogs and cats, but also more exotic pets such as birds and geckos. Also, according to the DOT report for 2017 , several of the animals had pre-existing health issues such as heart disease. (The report did not specify how many animals were flown in the cargo hold as opposed to the plane's cabin.)

But even when accounting for their larger volume of animal passengers, United still leads all other airlines in rate of deaths per 10,000 pets.

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United did not respond to multiple requests for comment for this story. But spokesman Charlie Hobart told CNN on Tuesday that the airline is investigating the French bulldog's death "to prevent this from ever happening again."

As for the 2017 report, he said, "The overwhelming majority of the incidents were attributed to animals not being acclimated to its crate or the animal having a pre-existing condition we weren't aware of. Any time there is an incident with animals we do a thorough review."