The grey whale that has delighted Japanese holidaymakers in recent days by swimming in Tokyo Bay has died.
Grey whales are rarely seen in Japan
It was found entangled in a fishing net near the town of Tomiyama in southern Chiba prefecture.
A Japanese government official said it was not yet clear how the 10m (33ft) whale perished.
Hideki Moronuki, in the Japanese Fisheries Agency, said scientific researchers were investigating the cause of death.
Mr Moronuki said he himself had two theories.
"It was trapped in the net and died because of entanglement... [or] it died before it was trapped by the net because of a crush with vessels or for other reasons, and then drifted into the net. But at this moment, nobody knows," he said.
He said local officials and fishing co-operatives would decide what to do with the animal once the tests had been completed.
The whale enthralled locals during Japan's Golden Week holiday
Whales are best known in Japan as a culinary delicacy, and each year Japan kills about 400 minke whales, as well as a smaller number of Bryde's whales, sei whales and sperm whales, in the name of research.
Critics say that Japan is using the excuse of scientific research into whales as a pretext to continue selling, and eating, whale meat.
But Mr Moronuki said his agency must be applied to for permission if the whale is to be sold as meat, and so far he had received no such requests.
"Everybody knows that the grey whale is a rare species... so I think many people have the same idea that the animal should be used for scientific purposes," he said.
According to an official at nearby Kamogawa Sea World aquarium, about 30,000 grey whales live in the Pacific Ocean.
They feed on marine worms and crustaceans, and can weigh up to 35 tonnes.