BOSTON (AP) – A rare, 350-pound sea turtle that stranded on a Cape Cod beach died Friday at the New England Aquarium.

The 5-foot female leatherback turtle, which was rescued from a beach in Dennis on Tuesday, appeared to have a parasite in its lower gastrointestinal tract and low blood-sugar levels. But the cause of the turtle’s death was a mystery, said aquarium spokesman Tony LaCasse.

Experts at the Woods Hole Oceanographic Institute planned to perform a necropsy and a full-body CAT scan on the turtle to pinpoint the root of its illness.

Veterinarians treated the turtle with antibiotics and intravenous fluids, but the animal remained in critical condition before it died.

Injuries from boat strikes or fishing gear entanglements are a common cause of death for leatherback turtles, which are considered a critically endangered species.

“This animal didn’t have either, which is what made the case so unusual,” LaCasse said.

However, officials are exploring whether the turtle’s illness resulted from ingesting a foreign object, such as a plastic bag or a balloon, according to LaCasse.

Leatherbacks are the world’s largest turtle, with some adults weighing more than 2,000 pounds. More than a dozen of them strand on Cape and Islands beaches every year, said Bob Prescott, director of the Massachusetts Audubon Society’s Wellfleet Bay Wildlife Society.

“Our record of saving them is getting better and better every year,” he added.

The leatherback turtle was the aquarium’s first in at least 30 years.

“They historically never do well (in captivity),” LaCasse said. “This is an open ocean turtle, and they have no cognition of boundaries.”

To prevent the turtle from crashing into the walls of its tank and injuring itself, aquarium officials hooked it up to a Velcro harness so it would swim in place.

“It worked out amazingly well,” LaCasse said.