A Houston-area family is mourning the death of its dog and demanding answers from United Airlines this week after its 5-year-old Cavalier King Charles spaniel was found dead in the cargo hold after their flight.

In a Facebook post, the Rasmussen family said her family pet, Lulu, was cleared for travel by a veterinarian before they dropped off the dog at animal cargo at George Bush Intercontinental Airport, along with another family dog.

Weather delayed the Sunday flight for nearly two hours, which the Rasmussens suspect was a factor in the death. It is unclear whether the dogs were in the cargo hold of the plane during the entirety of the flight delay.

“We are still waiting for answers,” the family posted on Facebook. “There were significant problems with the air conditioning in the main cabin where we are sitting, so we are deeply concerned about the conditions in cargo … and worried that there was significant negligence on the part of #UnitedAirlines.”

The airline released this statement:

“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 four-hour flight was scheduled to depart the city at about 1 p.m. but took off closer to 3 p.m., according to flight records. National Weather Service records indicate Sunday’s high temperature in Houston was 94 degrees with rain.

While Lulu was found dead upon arrival in San Francisco, her companion, Ginger, was released to the Rasmussen family in good health.

United has come under scrutiny in recent years after a series of high-profile incidents in which outraged owners have spoken out about the loss of their pets. In 2012, supermodel Maggie Rizer wrote a blog post accusing the carrier of negligence in the death of her 2-year-old golden retriever. A week later, a Florida man bemoaned the death of his mastiff due to heatstroke during a cross-country flight.

In what became United’s most trying public-relations month, Simon the giant rabbit died in April aboard a flight from London to Chicago. The three-foot-long rabbit was the progeny of a Guinness World Record-holding giant rabbit. The death followed a viral video that captured a passenger being forcibly removed from a United flight earlier in the month.

USA Today compiled data from U.S. Transportation Department reports that found that the carrier accounted for a third of all animal deaths aboard U.S. flights from 2012 to February 2017. In many cases, animals died from preexisting conditions, heart problems, injured themselves during flight or were in delicate health before the flight.

It’s unclear whether any of those were a factor in Lulu’s death.

The Rasmussens declined an interview request on Tuesday. On Facebook, the family said it is suffering.

“Lulu brought us so much comfort and love to our family and we can’t help but smile when we remember how she would snuggle up with her cav sisters … and get mad at us when we would try to brush knots out of her hair,” according to the post.

A United spokesman said the carrier is investigating the dog’s death. Airlines are required to provide details about such incidents in a monthly “Air Travel Consumer Report” to the federal government.