United Airlines on Wednesday said the owner of a puppy that died in one of its overhead bins told a flight attendant the dog was in the bag before it was stowed there. The flight attendant, however, did not hear or understand the woman, the airline said, which called the incident a "tragic accident."

The flight attendant "did not knowingly place the dog in the overhead bin," United spokeswoman Maggie Schmerin said in a statement.

The carrier declined to say whether the flight attendant was fired over the incident.

The Department of Transportation said it is investigating the incident and is in contact with the Department of Agriculture, which handles alleged animal mistreatment complaints.

Sen. John Kennedy of Louisiana said he planned to file a bill on Thursday that would ban dogs in overhead bins, saying "violators will face significant fines." The union representing United flight attendants said no flight attendant would knowingly instruct a passenger to put an animal in an overhead bin.

I will be filing a bill tomorrow that will prohibit airlines from putting animals in overhead bins. Violators will face significant fines. Pets are family

Kennedy also wrote a letter to United's president, Scott Kirby, asking United to explain a string of pet deaths on its planes.

The French bulldog puppy was in a carrier that was sticking out into the aisle when a flight attendant told the passenger it would have to be moved, according to people familiar with the incident. Aisles must be clear for safety reasons.