Close to 100 false killer whales were killed early last week after a large pod trapped itself in a mangrove forest off the coast of Florida’s Everglade National Park. The National Oceanic and Atmospheric Administration reports that false killer whales are currently not an endangered species, however, its Hawaiian population has recently come under threat of extinction.

NOAA, the Florida Fish and Wildlife Conservation Commission, the U.S. Coast Guard, and the Mote Marine Laboratory collaborated in an effort to rescue the whales. Rescuers on boats attempted to herd some of the dolphins from shallow muddy water to deeper clearer water.

Sadly, many of the marine mammals were too immersed within the mangroves and therefore unreachable to the rescuers. Overall, 88 false killer whales are believed to be dead, making this the largest stranding of the species in U.S. history. Scientists are still unsure of what led these whales to swim so close to shore.

False killer whales are a part of the dolphin family. They closely resemble their namesake, the killer whale, except for their all-black skin pattern.