FILE PHOTO: Stranded false killer whales are seen on a beach at sunrise, at Kommetjie near Cape Town May 30, 2009. REUTERS/Chad Chapman

(Reuters) - Biologists on Tuesday were investigating the weekend deaths of dozens of dolphins in Everglades National Park in Florida’s largest mass stranding of the mammal since 1989, a U.S. scientific agency said.

At least 82 of the dolphins, known as false killer whales for their resemblance to killer whales, died, the National Oceanic and Atmospheric Administration (NOAA) said. Seventy-two were found dead and 10 were euthanized by veterinarians on the scene after being found in poor condition.

Thirteen dolphins remain unaccounted for since the initial sighting Saturday afternoon by a bystander in Hog Key, a remote island on the western side of the park, NOAA spokeswoman Blair Mase said.

The dolphins were scattered along the hard-to-reach shoreline and deeply embedded in some of the mangroves, making it hard to retrieve them, said Mase, coordinator of NOAA’s Southeast Marine Mammal Stranding Network.

“Once on the scene, the response team attempted to herd some of the free-swimming live whales into deeper water, however, they were ultimately unsuccessful with that effort,” Mase said.

False killer whales, the fourth-largest dolphin, range in size from 15 to 20 feet (4.6 to 6.1 m) long, weigh about 1,500 pounds (680 kg) and have a 60-year lifespan, according to NOAA’s website.

Approximately 28 false killer whales were discovered stranded in Key West in 1986.

Biologists and responders from numerous agencies including NOAA, the Florida Fish and Wildlife Conservation Commission and the National Park Service will try to find the unaccounted dolphins in Everglades National Park and determine the cause of death using samples collected during post-mortem examination.