The tragic incident went down on July 26 after 9-month-old French Bulldog boarded flight KL617 from Amsterdam to Boston. The family's attorney, Evan Oshan, tells TMZ ... the family's father was shipping his 6-year-old son's dog to their home in the states. Oshan says when the boy's grandmother went to pick up the dog at the airport, officials broke the tragic news to her.

The family had a necropsy performed on Roger and according to the docs provided by Oshan, Roger died of heat exhaustion and dehydration. The cause of death is listed as respiratory distress/heat stroke. The report also says "limited panting in a stressful, hot or humid environment, combined with a lack of water, would have placed Roger at high risk for heat stroke."

The family now wants answers and has hired Oshan, who has sadly dealt with similar cases. As we first reported ... a Central Asian Shepherd was found dead back in in March in the cargo hold of a KLM flight from Amsterdam to L.A.

Oshan confronted airline officials at LAX demanding answers. At the time he got none ... but vowed to get to the bottom of it.