Of all U.S. airlines, United had the highest rate of pet deaths in 2017. And in 2016. And in 2015.

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. 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.

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.”

Meanwhile, United has announced it will issue special bag tags for animal carriers. The airline said a flight attendant who ordered the passenger to put her pet carrier in the overhead bin aboard a Houston-to-New York flight Monday didn’t know there was a dog inside.

“To prevent this from happening again, by April we will issue bright colored bag tags to customers traveling with in-cabin pets,” United said in a statement.

The family that owned the dog and other passengers contradicted the airline’s account, saying the dog’s barks were audible from inside the bin.

Eleven-year-old Sophia Ceballos told NBC News that her mother told the flight attendant “‘It’s a dog, it’s a dog,’ and (the flight attendant) said we have to put it up there,” in the bin.

Other passengers backed up the family’s account on Twitter and Facebook.