One bottlenose dolphin died and its calf was rushed to a rehabilitation hospital Wednesday after a rescue effort aimed at returning them from the Shrewsbury River to the Atlantic Ocean failed.

The mother had been loaded into a truck for a one-mile trip from the riverbank to the beach, where she was to be released. The cause of death wasn't known, nor were the age and sex of the calf.

The calf was immediately transferred into a truck and was being taken -- with a New Jersey State Police escort -- to the National Aquarium in Baltimore, according to Teri Frady, a spokeswoman for the National Oceanic and Atmospheric Administration, which participated in the rescue effort.

In early September, the dolphins swam into the river from the Atlantic Ocean, probably in the hunt for menhaden or other forage fish. But they failed to return to the ocean.

The National Oceanic and Atmospheric Administration has organized a 30-person team of veterinarians, mammal handlers, divers and marine biologists to attempt the rescue, oceanic administration spokeswoman Teri Frady said Tuesday.

"We knew that if we didn't move them, this would be the result anyway. It's a high-risk operation. Even though we have some of the best people in the country, you can't always count on success," she said.

A necropsy was scheduled to determine what killed the dolphin.