Environmental officials found hundreds of dead sea turtles floating off the coast of El Salvador this week, prompting concern about what may have caused the mass die off. El Salvador’s Environmental Ministry took to Twitter Thursday to confirm the find of between 300 and 400 dead turtles.

The turtles were found floating about eight miles from Jiquilisco Bay. Most were decomposing when environmental officials found them. It remained unclear what species the turtles were.

“We don’t know what caused the sea turtles’ death,” the ministry said. “We collected samples from the dead turtles. They will be analyzed in a laboratory to determine what killed them.”

Officials surmised the turtles may have died as a result of toxic algae blooms known as red tides.  In a similar situation in 2013, 114 sea turtles washed up from late September through October on beaches in El Salvador, the environmental ministry said. Hypotheses at the time said the turtles were killed by saxitoxin, a neurotoxin often found in red tides. Saxitoxin reportedly killed 500 turtles in El Salvador in 2006 and 100 in the same region in 2010.

Red tides produce an abundance of deadly toxins. The algae blooms often appear blanket-like in the ocean, covering the food sources of turtles. Turtles and manatees swallow the toxins along with their food or inhale them, according to the Smithsonian.

Seven different species of sea turtle exist in the world’s ocean, including the green, hawksbill, loggerhead, leatherback, olive ridley, Kemp’s ridley and flatback. Almost all species of the turtles are listed as endangered with the World Wildlife Foundation. In addition to the dangers presented by toxic algae blooms, sea turtles are exploited by humans, slaughtered for their eggs, meat and shells and in danger of habitat destruction and accidental capture by fishermen.