An “unusual mortality event” is unfolding off the East Coast of the United States for humpback whales, and experts are trying to get to the bottom of what is happening. A normal year sees about 14 humpbacks wash up on East Coast beaches, but in the last 16 months at total of 41 dead whales have ended up on U.S. shores.

Authorities want to know why the humpback whales are dying at such an alarming pace, and they are conducting complete physicals of whale caracasses for clues. While ship strikes have seen a noticeable increase lately, many of them appear to be dying from some type of illness, according to reports.

An “unusual mortality event” is declared if the number of strandings exceed the annual average for the Greater Atlantic Region. This region extends from Virginia to Maine along the East Coast. A total of 15 of the 41 deaths have occurred in 2017 alone.