United Airlines apologized Tuesday for forcing a passenger to stow away an in-cabin dog inside an overhead bin, causing the pet to die.

“This was a tragic accident that should never have occurred, as pets should never be placed in the overhead bin,” Maggie Schmerin, a spokeswoman for the airline, said in a statement, New York Times reported. “We assume full responsibility for this tragedy and express our deepest condolences to the family and are committed to supporting them.”

The incident occurred inside a United Flight 1284 from Houston, Texas, to New York on Monday, when the flight attendant spotted a woman tucking a small carrying bag under her seat. She was requested by the attendant to the keep the bag in one of the overhead bins.

Despite informing that the bag contained her pet dog, the attendant kept on insisting that the bag be transported to the overhead bin. So the woman reluctantly let her 10-month-old dog be stowed away in the overhead bin.

“At the end of the flight, the woman found her dog, deceased. She sat in the airplane aisle on the floor crying, and all of surrounding passengers were utterly stunned,” Maggie Gremminger, 30, a fellow passenger wrote on Twitter, posting a picture of the distressed woman and her daughter.

However, as far as the United Airlines policy on in-cabin pets goes, the passenger might not have been complying with all the mentioned rules.

While some reports stated the woman was carrying the dog in a pet carrier, many others reported she was carrying it in a carrying bag. If it is in fact the latter, the woman was in violation of one of the crucial policy rules.

“A pet traveling in cabin must be carried in an approved hard-sided or soft-sided kennel. The kennel must fit completely under the seat in front of the customer,” states the United Airlines in-cabin policy.

Gremminger also said the woman was preoccupied with her infant and was wholly unaware of the fact that her dog — which was barking loudly at the time of takeoff, fell silent as the plane continued to gain altitude.

Although the United Airlines has kept mum on whether the flight attendant mishandled the situation, the policy clearly mentions that pet carriers “must fit completely under the seat in front of the customer and remain there at all times,” which means it is against the airlines’ policy to tuck pets inside overhead bins.

One of the argument that the flight attendant gave for putting the dog away in the overhead bin was that the passenger’s pet carrier was sticking out from under the seat, which could cause someone walking down the aisle to trip and fall. This might mean the pet carrier was larger than the recommended size in the policy.

As investigation continues on the incident, United Airlines has refunded the passenger’s ticket fare along with the in-cabin pet fee of $125.