Reports of a "wolf-dog" hybrid had environmental officials investigating in Ledyard and North Stonington.
Officials in those towns said the animal was attacking people and horses.
The Department of Energy and Environmental Protection was investigating.
DEEP said the reports claimed the animal was spotted around the Long Pond area in Ledyard and around popular hiking spots in North Stonington.
They attacked at least two horses, bit a bicyclist and threatened a resident who shot and killed one, according to people in those towns.
Connecticut state law says ownership of a hybrid animal is illegal. DEEP said it has the authority to seize the animals, if found.
DEEP was expected to provide an update on its investigation Tuesday afternoon.
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