A third harbor seal attacked by a mysterious slasher has died.

The OKEANOS Ocean Research Foundation said the seal, found Jan. 9, was thought to be getting better but had a stroke and died Sunday.

It was the third harbor seal to die from stab wounds in the last three months. The first was found dead on the shore of Staten Island on Nov. 8. The second washed ashore on Long Island on Nov. 29 and died a few days later.

All the stab wounds were to the lower body. Kim Durham, a biologist for the foundation, said the wounds were not characteristic of boat propellers.

A $10,000 reward has been offered for information leading to the arrest of those responsible for stabbing the seals.