California (USA) - An animal rescue organization from the US state of California is worried: several whales have died near San Francisco Bay within a very short space of time. But determining the causes of death presents researchers with an impossible task.

Their most recently discovered victim is a rare minke whale. According to the Marine Mammal Center, there have only been five sightings of this species in the last 16 years.

The rare specimen was still swimming on the coast last Tuesday when staff noticed that the animal had sunk into the mud at low tide.

For the time being, the minke whale was able to free itself and return to deeper waters, according to the organization on X. However, shortly afterwards it got caught in the shallow water again and could not be freed by helpers.

With little chance of survival, the decision was made to free the marine animal from its agony and "humanely euthanize" it.

The Marine Mammal Center emphasizes in its post that euthanizing the animal was not the desired outcome, but it was done in the best interest of the minke whale.

While the unfortunate circumstances of the minke whale's death are clear, the organization is faced with a mystery regarding three other whales that were recovered dead.

Since the end of March, helpers have recovered two male and one female gray whale. Although the latter was malnourished, the circumstances that led to the animal's death are still unclear.

The two male marine giants also showed no signs of blunt force trauma, for example due to an accident with a ship. Here too, the organization is puzzling over the causes of death.

However, one of the animals had broken vertebrae, which had led to bleeding. Detailed investigations in the coming weeks will show whether this is the final cause.