An outing to the Pittsburgh Zoo turned into tragedy on Sunday when a woman put her young son on top of a railing so he could get a better look at some wild African dogs.

Officials are still trying to piece together exactly what happened, but the mother apparently stood her son up on top of a wooden railing over the pen and was unable to catch him when he lost his balance. He fell 12-14 feet into the enclosure and was immediately attacked by the animals–sometimes called African Painted Dogs–as he lay prone. Police say they are awaiting an autopsy to verify whether the boy died on impact or if his death was caused by the mauling.

The animals–11 in all–were reigned in by zoo keepers except for one, which was so aggressive it had to be put down on the spot. Bystanders said they heard screams and the shots that rang out when that animal was killed, and describe it as a horrible tragedy.

“They are one of the most aggressive predatory animals in the wild,” animal expert Jack Hanna said. “I don’t care if a zookeeper, a policeman, a tranquilizer gun, whatever would have been right there, could not have helped. Sorry is not a word that I can even say…I don’t know what word to use. Just, my heart aches for everybody.”