After most of the 51 pilot whales that were stranded on the edge of the Florida Everglades National Park last week swam back into deeper waters, 11 more whales believed to be from the same pod were found dead in the lower Florida Keys on Sunday, officials said, adding that chances of survival for the remaining whales were very slim.

Despite rescue efforts by scientists, 10 of the whales died on Wednesday and another whale was found dead Thursday. Now, with 11 whales found dead on Snipe Point, about six miles north of Sugar Loaf Key, the death toll has increased to 22.

“We believe these are part of the original pod of 51 stranded pilot whales,” National Oceanic and Atmospheric Administration's Fisheries Service said, in a statement. “Given our knowledge of past mass pilot whales strandings, the outlook for finding the remaining whales alive is bleak.”

Officials said that 29 whales remained unaccounted for ever since they were first seen alive on Friday, and feared that the remaining whales may already be dead. Pilot whales, which are known to be highly social and live in a cohesive unit, are most susceptible to end up in mass strandings as they are reluctant to break away from other members of their pod even in trouble.

While experts are yet to offer an explanation for the mysterious deaths of the whales in the Florida Keys, scientists are expected to take samples of the 11 whales that died on Monday to determine cause of death.

Scientists also have performed necropsies on the 11 deceased whales to look for possible diseases and pathogens as well as environmental and human causes. However, results of testing are not expected to arrive for at least several weeks. They will also look for the presence of biotoxins in the pod.

According to the U.S. Coast Guard, while there was no evidence of sonar trauma, they are yet to hear from the U.S. Navy. The 11 dead whales varied in sex and age from juveniles to adults, and their empty stomachs indicated that their health might have been compromised prior to the beaching.

This is not the first incident of a whale stranding in Florida. In 2012, more than 20 pilot whales stranded themselves in Fort Pierce, and only five were rescued.