Sen. John Kennedy John Neely KennedyMORE (R-La.) is pressing United Airlines for details amid backlash over a dog who died after a flight attendant told the owners to place the pet in an overhead bin.

"I write to demand an immediate explanation for the number of animals who have died recently in United Airlines' care," Kennedy wrote in a letter to the airline's president, Scott Kirby, on Wednesday.

The senator cited a statistic from the Department of Transportation, saying 18 of the 24 pets who died on major airlines last year were in the care of United. Another 13 animals suffered injuries on United flights, while Delta and American each reported two animal deaths.

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"This pattern of animal deaths and injuries is simply inexcusable. For many people, pets are a member of the family. They should not be treated like insignificant cargo. Frankly, they shouldn't be placed in the cargo hold, much less an overhead bin," Kennedy wrote.

Catalina Robledo and her 11-year-old daughter Sophia Ceballos placed their French bulldog Kokito in a carrier but struggled to place him under the seat. A flight attendant then told the two to place Kokito in the overhead bin.

United Airlines issued a statement on Tuesday, taking responsibility for the incident.

"We assume full responsibility for this tragedy and express our deepest condolences to the family and are committed to supporting them. We are thoroughly investigating what occurred to prevent this from ever happening again," a spokeswoman said in a statement.

The airline is also under fire for flying a German Shepherd to Japan instead of Kansas City, Mo., with its family after the airline confused the German Shepherd's kennel with a Great Dane's kennel. United said it is investigating the incident.

United also faced major public backlash last year after a viral video showed a passenger being forcibly dragged off of one of its flights.