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For the second time in recent weeks, an endangered leatherback sea turtle that was found stranded on Cape Cod has died, according to the New England Aquarium.

The cause of death for the turtle, a 286-pound juvenile that was critically ill and hypothermic when it was found Sunday, has yet to be determined. It died Tuesday after “extensive treatment” by the aquarium’s staff, according to a statement from the group.

Immediate results from a necropsy that was performed at a Quincy sea turtle hospital proved to be inconclusive, and a pathology report is expected in coming weeks, according to the aquarium.

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It is the second live leatherback sea turtle found stranded on Cape Cod during the last two weeks, something the aquarium called “an unprecedented event.”

“We’ve had only five live leatherback strandings that we’ve worked on since 2005 — five in 13 years — so having two live leatherback strandings in two weeks is unusual,” said Dr. Charles Innis, the aquarium’s chief veterinarian, in a statement.

When the turtles are found stranded, they are typically dead, according to the aquarium.

On Halloween, a 420-pound female leatherback was found stranded in Brewster and died following treatment in Quincy.

For that turtle, the cause of death was determined to be complications related to a severe injury from entanglement as well as plastics that were found in its stomach, according to the aquarium.

Entanglements and boat strikes, particularly in the summer and fall, are common causes of death for leatherbacks, according to the aquarium, while plastic ingestion is becoming increasingly a problem for the species. Some turtles mistake the plastic for sea jellies, the aquarium said in a statement.

Innis said the aquarium has had “many leatherback sightings in the area over the past few months, but this case is different because we don’t have a clear entanglement or boat strike as the cause of death.”

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During the last 13 years, at least 220 endangered leatherbacks have been entangled in marine ropes along the Massachusetts coast, according to the aquarium. There have been a dozen known entanglements this year, the group said in a statement.


Danny McDonald can be reached at daniel.mcdonald@globe.com. Follow him on Twitter @Danny__McDonald.