When the Rasmussen family landed in San Francisco on Sunday, they learned that family dog Lulu had died during the flight despite Lulu being cleared by a veterinarian to fly, ABC News reported. 

The family flew from Houston to San Francisco, and their 5-year-old Cavalier King Charles Spaniel was stored in the cargo hold. The family said that the flight was stuck on the tarmac for two hours in Houston before taking off. 

The family added that the cabin had air conditioning problems, but did not know if either issue contributed to Lulu's death. 

"We are still waiting for answers, but what we do know is that our flight was delayed on the runway in Houston for more almost two hours," the family said.

A representative from United Airlines issued a statement to ABC News.

"We are so sorry to learn of Lulu’s passing and have reached out to our customer to offer our condolences and assistance. We are deeply upset any time an animal suffers an injury while traveling with us and especially grieved in the rare instance that one passes away. We are conducting a thorough review of this incident," the statement said.

According to ABC News, one-third of all animal deaths on U.S. carriers occurred on United Airlines, since 2012, citing Department of Transportation data.