Whale beachings aren't rare on the Chilean coast, but this is the first time sei whales have been found there in this matter.
Scientists and law enforcement officials are still working to rule out foul play, but they're now sure that the whales found didn't die at the same time. In fact, they can determine at least three times of death within the group. The most likely cause of a mass death like this one (where no injuries are found) is some kind of toxic algae bloom. In fact, that has been pegged as the likely cause of death for related whales recently found in a 5 million-year-old Chilean mass grave.
When algae that produces toxins overtakes a body of water, those toxins make it into the food chain by way of the tiny creatures that feed on it. In the case of these sei whales, the scientists studying them hypothesize, sardines poisoned by the algae could have given the whales deadly food poisoning.
But it's also possible that the whales were killed by a virus, in which case it will need to be determined whether other members of the population are still at risk.