The federal government is launching an investigation into the recent deaths of more than a dozen North Atlantic right whales.

At least 13 of the whales have been found dead this year off the coasts of Canada and New England. The whales are among the rarest marine mammals in the world, numbering no more than 500.

An arm of the National Oceanic and Atmospheric Administration said Thursday that it is declaring the deaths “an unusual mortality event.” The agency says that designation triggers a “focused, expert investigation” into the cause of the deaths.

Representatives from Fisheries and Oceans Canada and NOAA Fisheries are expected to provide more information about the investigation Friday morning. Some of the whales have died from ship strikes or fishing gear entanglement.