As many as 50 manatees have died, floating to the surface of rivers and lagoons in Mexico’s southern state of Tabasco, where some of the country’s major oil wells have been operating for decades. Fish, reptiles, birds and other marine animals have also died suddenly, and scientists are scrambling to figure out the cause.

“It could be one of the greatest environmental emergencies we have had in recent times,” Ernesto Zazueta, president of the Association of Zoos, Breeders and Aquariums of Mexico told El Universal newspaper.

On May 18, residents of the municipality of Bitzales found a lifeless manatee floating in the water. They assumed it was an isolated incident, or that the animal had been sick. But in the weeks the followed, more and more dead manatees were found in the state’s waterways. Fishermen reported that suddenly they weren’t catching anything, and dead fish also began floating to the surface.

Officials have sampled the water and carried out necropsies on the animals, but the cause of the die-off remains a mystery. One study from the Aquatic Resources Research Laboratory at the Technical University of Boca del Rio found high levels of lead and cadmium in the water. But other biologists said toxic substances have not been found in their tests of the water and the manatees.

Another study commissioned by the federal government and the National Autonomous University of Mexico said the die-off was possibly the result of a range of factors, including rising water temperatures.

For about 40 years, Mexico’s state oil company, Petróleos Mexicanos, has operated oil wells and gas facilities in the vicinity of the die-off. Locals have begun to blame the company for the crisis, which has also affected the livelihoods of thousands of fishermen in the state.

PEMEX has denied responsibility, saying that it sent experts to the site to investigate.

“There is no (PEMEX) activity adjacent to the tributaries where the problems have been identified,” the company said in a statement. It added that the facilities in the area are “operating normally, without any record of a leak or spill.”

But a journalist for El Universal who reported from the scene wrote that there are two Pemex oil wells in the vicinity of the affected area.