Weeks of tests have failed to find the cause of the mysterious mass die-off of thousands of fish washing up on the southwestern shores of Nova Scotia.

Besides a tsunami of dead and dying herring, creatures that have washed ashore include lobsters, starfish, scallops, crabs, clams and apparently a humpback whale, Canadian TV reported. The number of species impacted has gradually increased over the month, and up to 20,000 animals have turned up dead. Though tests haven’t found any known toxin or disease, none of the dead fish and shellfish are safe to eat, authorities warn.

“Dead or dying herring found on shore should not be collected, consumed or used by the public for any reason, as a variety of factors could affect the food safety of fish, such as toxins, diseases or environmental contaminants,” warned the Canadian Food Inspection Agency and the Department of Fisheries and Oceans in a statement.

Herring found by DFO C&P Officers in Southwest Nova Scotia yesterday was not fresh (4-5 days old) pic.twitter.com/CyQzxwxQrH — DFO Maritimes (@DFO_MAR) December 29, 2016

Nothing has been found to explain the horrifying scenes on the shores. Officials have tested for infections, diseases, toxins, predators and some water quality issues.

Some scientists speculate that the die-off could be related to low oxygen levels from pollution, agricultural runoff, low salinity from heavy rains, or possibly some complication caused by a new five-story underwater turbine built to generate power.

“We’re kind of in the dark, not from lack of trying, but from the complexity of the case,” Ted Leighton, an adjunct biology professor at Nova Scotia’s University of Sainte-Anne, told Metro News. There are “no firm data to rule anything in or out.”