On July 24, 26 dead sea turtles washed onto a beach in Puerto Arista, a small fishing and tourist town in Mexico’s Chiapas state. Over the next three weeks, as Reuters reports, dozens more turtle carcasses continued to wash ashore—a total of 113 dead turtles had been found as of August 13. And experts still don’t know what is causing the die-off.

The Pacific waters off the coast of Puerto Arista are part of a marine sanctuary, and all of the dead turtles are classified as “endangered by extinction” under Mexican guidelines, according to a statement by Mexico’s Federal Attorney for Environmental Protection (PROFEPA). Of the 113 individuals that have been found, 102 are olive ridley turtles, six are hawksbills, and five are Pacific green turtles.

Their bodies were discovered across a 19-mile stretch of beaches. Kimberly Hickok of Live Science reports that the turtles were all adults, both male and female. Some, according to the Riviera Maya News, were likely between 80 and 100 years old. The bodies of the dead turtles were in varying states of decomposition.

Just what has killed these unfortunate creatures is unclear, and officials are in the process of taking water samples and conducting necropsies on the dead turtles. It is possible, according to PROFEPA, that a noxious algal bloom is to blame for their deaths. Under certain conditions—like warm temperatures and surges of nutrients in the water—colonies of algae can proliferate and become harmful. Florida, in fact, is currently suffering from a toxic red algal bloom that has killed hundreds of sea creatures, including 300 sea turtles. Hickok of Live Science reports that a similar algal toxin could be contributing to the deaths of the turtles in Mexico.

But officials suspect that something else might be at play. Some of the turtles found on July 24 had injuries that seem to have been caused by hooks or fishing nets, leading the authorities to believe that they may have become entangled in legal fishing nets in the area.

For now, while the results of the water sample tests and necropsies are still pending, federal and state authorities will be monitoring the beaches of Puerto Arista and working to encourage local fishers to engage in practices that will keep sea turtles safe.