According to passenger Maggie Gremminger, she and others heard the black French bulldog barking initially during the flight, and were horrified to learn the animal had passed away later in the trip, she told People.
'They INSISTED that the puppy be locked up for three hours without any kind of airflow.
The dog suffocated to death during the three-hour flight, and United Airlines says it takes full responsibility while it investigates the accident.
Another passenger, June Lara, wrote in a public Facebook post of the moment it was clear the dog had died.
Once the plane had landed, Lara said the dog was unresponsive. "The kennel must fit completely under the seat in front of the customer and remain there at all times". "After the flight attendant asked her to move it above, the woman adamantly refused, communicating her dog was in the bag".More news: QB Case Keenum intends to sign with Broncos, sources say
The owner, she said, appeared "absolutely shocked and heartbroken".
The French bulldog was traveling Monday with its owner and her children from Houston to NY.
"We assume full responsibility for this tragedy and express our deepest condolences to the family and are committed to supporting them", the airline said in a statement.
"Another reason I will never ever fly or support doing business with United Airlines!"
A spokesperson for United could not say if anyone had been disciplined in relation to the incident. According to Department of Transportation statistics on animal incidents on USA carriers, United has the highest rate of incidents involving loss, injury or death of animals during air transportation, with 2.24 incidents every 10,000 animals transported.