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Eleven pilot whales were found dead Sunday in the lower Florida Keys, doubling the number of dead whales found in the past week, CNN reported.

Officials with the National Oceanic and Atmospheric Administration believe the 11 whales were part of a pod of 51 whales stranded on the edge of Everglades National Park.

Scientists are working to determine their cause of death.

"This totals 22 dead whales and 29 still missing," NOAA said in a statement. "Given our knowledge of past mass pilot whales strandings, the outlook for finding the remaining whales alive is bleak."

The last time the group was spotted alive was Friday. Then, a pod of 20 whales was seen five nautical miles offshore, moving inshore and in a southward direction.

They were reportedly swimming slowly and in a disorganized fashion, which could have suggested exhaustion, dehydration or malnutrition, according to NOAA.

Pilot whales are protected under the Marine Mammal Protection Act, but they are not endangered.

Mass strandings by pilot whales are not altogether uncommon in Florida.

In 2012, nearly two dozen pilot whales stranded and beached themselves. In 2011, 23 pilot whales stranded and beached themselves.