United Airlines has apologised to a family who had claimed that their dog had died in the cargo hold during a flight from Houston to San Francisco. The airline has had the highest number of animal deaths in the country’s air transport history, according to Reuters.

The Rasmussen family from Houston had said in a Facebook post that there had been trouble with the air conditioning in the cabin when the flight was delayed because of bad weather on Sunday. Lulu, a five-year-old Cavalier King Charles Spaniel, was found dead after the flight arrived in San Francisco.

The family had said that the dog was cleared for flying by a veterinarian just days before the flight, reported ABC News. The Facebook post was, however, later deleted.

In its statement, United Airlines said they were reviewing the incident. “We are sorry to learn of Lulu’s passing and have reached out to our customer to offer our condolences and assistance,” United said in a statement, according to CBS. “We are deeply upset any time an animal suffered an injury while travelling with us and especially grieved in the rare instance that one passes away.”

In April this year, Simon, believed to be the world’s largest rabbit, had died on a transatlantic United Airlines flight from London to Chicago. The animal’s investors had sued the airline after the incident.