Big Pine Key, Fla. ? At least 28 pilot whales stranded themselves Friday in shallow water off the Florida Keys, and five have died, officials said.

The whales were stuck in two separate groups about five miles north of Big Pine Key, said Laura Engleby of the National Oceanic and Atmospheric Administration Fisheries Service.

Seven of the surviving whales were transported to a temporary holding facility in the lower keys, where they will be evaluated for release. Other survivors were moved into deeper water in hopes they would begin swimming on their own, Engleby said.

“But we won’t be surprised if they strand themselves again,” she said.

Volunteers and veterinarians who spent most of the day caring for the whales left the water at dusk, concerned about sharks.

“It’s just impossible to leave anyone here overnight,” Engleby said.

Rescuers will return this morning. With some of the whales much sicker than others, Engleby said, marine veterinarians will have to decide the most humane treatment if the whales strand themselves again overnight.

Rescuers helped load the seven whales onto boats for relocation. A barge from a towing and salvage company was assisting with the rescue and recovery effort.

The stranded pods include adult males and females as well as calves. Some of the whales are as large as 17 feet in length and weigh as much as several tons, Engleby said.