United Airlines has taken “full responsibility” after a dog died in-flight after a flight attendant "insisted" on the pet being placed in an overhead locker. The US airline apologised for the death of a family’s black French bulldog on a flight from Houston to New York on Monday and has launched an investigation. Passenger Maggie Gremminger, 30, who was seated behind the pet owner and her two children, said she witnessed a United flight attendant insisting on the canine being placed in the overhead locker in its pet carrier. “Today I was on a plane where I witnessed a United flight attendant instruct a passenger to place her dog carrier in the overhead compartment,” she wrote on social media. “The passenger adamantly refused but the flight attendant went on with the instruction. “At the end of the flight – the dog was found dead in the carrier. I am heartbroken right now. I didn’t question the flight attendant but I could have. “I assumed there must be ventilation as surely the flight attendant wouldn’t have instructed this otherwise. I heard the dog barking a little and we didn’t know it was barking a cry for help.” This should never have happened. How did this happen? I am not sleeping tonight. @nytimes@washingtonpost@WNYCpic.twitter.com/5x5bckI7EU— MaggieGremminger (@MaggieGrem) March 13, 2018 Another passenger, June Lara, from Texas, said flight attendants “insisted” the puppy was locked up for the three hour flight. “They assured the safety of the family's pet so wearily, the mother agreed,” she wrote on Facebook. “There was no sound as we landed and opened his kennel. There was no movement as his family called his name. I held her baby as the mother attempted to resuscitate their 10-month-old puppy. “I cried with them three minutes later as she sobbed over his lifeless body. My heart broke with theirs as I realised he was gone.” United Airlines – worst PR disasters gallery United Airlines said the dog’s death was a “tragic accident that should never have occurred”, adding that pets “should never be placed in the overhead bin”. “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 a statement. “We are thoroughly investigating what occurred to prevent this from ever happening again.” United Airlines has taken "full responsibility" for the dog's death Credit: AFP The airline’s pet policy states they 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 and remain there at all times,” it adds. United Airlines came under fire last year after a passenger was dragged from an overbooked Chicago flight.