The dog was heard barking from the overhead bin by passengers 30 minutes into the flight before the pet fell silent

Maggie Gremminger is still heartbroken over what she witnessed on March 12 on United Airlines flight #1284 from Houston, Texas, to New York City’s Laguardia Airport.

The community relations manager tells PEOPLE that a fellow passenger’s dog died on the flight after a flight attendant adamantly instructed the unnamed passenger to store her carrier, with the pet inside, in an overhead storage bin for the duration of the flight. According to Gremminger, 30, the black French bulldog barked from the bin at least 30 minutes into the flight, but was found dead when the owner went to retrieve her dog once the plane had landed.

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The tragedy began when the dog owner boarded the plane with a young girl, a baby and the dog in a carrier, which Gremminger says looked like dog carriers that she has seen others use on planes. Gremminger was seated in the row directly behind the party of three and saw the woman place the carrier with the dog inside under the seat in front of her.

Shortly after she was seated, a flight attendant approached the woman with the dog and told her that the bag needed to be moved.

“The flight attendant told the passenger that her bag was blocking part of the aisle. I could not see it, as I was already in my seat, but it sounded like it was somehow not completely fitting beneath the seat in front of her,” Gremminger says of the encounter. “After the flight attendant asked her to move it above, the woman adamantly refused, communicating her dog was in the bag. There was some back and forth before finally the flight attendant convinced her to move the carrier to the bin above.”

After watching the woman stow her dog, Gremminger says she talked with her seat partner about how “stunned” they were to see a pet be placed in the overhead bin. At this point, Gremminger started to look up information online about whether overhead bins were safe for pets, but was unable to find any information before she had to turn off her phone for takeoff.

While the situation seemed off to Gremminger, she can understand why the owner ultimately decided to follow the flight attendant’s instruction.

“My only thought is that if it had been me, it would have been a hard scenario. The flight attendant is the authority figure, who should be trusted. I was thinking ‘maybe there is an improved ventilation system’ or something of the sorts,” Gremminger says. “Also, the owner had an infant and other daughter. Causing a scene before flight could risk being kicked off the flight. I can only imagine she felt stuck in her decision to comply.”

What Gremminger says haunts her the most about this experience was hearing the canine call out from the overhead bin, once while the plane was taking off and again during several moments of turbulence about thirty minutes into the flight. After this the dog was silent.

At the end of the flight the owner opened the overhead bin and placed the dog’s carrier on the floor. She soon realized that her pet had died during the flight and was “absolutely shocked and heartbroken.”

“A stranger offered to hold her newborn while she sat on the floor, there in the airplane aisle. She was holding her dog and rocking back and forth. Her daughter was also crying,” Gremminger says of the scene following the devastating discovery. “People who could not see what was happening were confused as to what was going on. I tried to encourage people to let them off first but it was confusing and so we waited for the rows to filter out. It was absolutely horrible.”

The crew, including the flight attendant who instructed the passenger to stow her dog, responded immediately to the woman’s distress. Gremminger says the flight attendant who moved the dog seemed “frazzled” and claimed that she was unaware a live animal was in the carrier.

“Many other crew members were contacting additional help and offering a blanket to the young girl who seemed cold. They were confused at how that individual flight attendant could have done this, but did not seem to take any sides or blindly defend,” Gremminger adds. “They were professional and did a wonderful job gathering information and being as supportive to the mourning family as possible.”

Image zoom Gary Hershorn/Getty

PEOPLE reached out to United Airlines for comment on Gremminger’s account of the events, and airline spokesperson Maggie Schmerin provided the below statement in response.

“This was a tragic accident that should never have occurred, as 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. We are thoroughly investigating what occurred to prevent this from ever happening again.”

According to United’s pet policy online, non-service dogs are permitted in the cabin with a service fee of $125 as long as the dog “is 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.”