(CNN) For more than three hours, a passenger's dog was inside an overhead bin on United Airlines Flight 1284 from Houston to New York after a flight attendant told the passenger to put it there.

Airline officials say it was a mistake. It turned out to be a deadly one.

When the plane landed at LaGuardia Airport Monday night, the dog was deceased.

In a statement, United called the dog's death a "tragic accident." Spokesman Charlie Hobart told CNN a flight attendant should not have told the passenger to put the dog in the bin used for carry-on bags.

"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 reply Tuesday to CNN's request for comment about reports of the dog's death. "We are thoroughly investigating what occurred to prevent this from ever happening again."

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