PARIS (Reuters). An ailing killer Whale stranded on the French River Seine in pain will be euthanized. A plan to return it to sea had failed, and scientists determined that it was suffering from terminal illness.

The male orca was identified at 4m (13ft) when it was first seen at the Seine mouth on May 16, between Le Havre in Normandy and Honfleur, Normandy. It then swam hundreds of miles downstream to reach the west side of Rouen.

The local prefecture met with international and national scientists to discuss how to bring the whale to safety. They also used a drone to emit orca sounds while guiding the whale to its final destination.

The whale was unable to respond “erratically” or “incoherently,” according to the Seine maritime prefecture. They recorded the whale sounding distress calls, which they described as similar to.

It stated that the attempt to return the whale to the sea had failed and it was decided to end the night’s intervention.

Researchers reviewed the data, photos, and videos from the intervention to determine that the animal had mucormycosis or black fungal disease. It was a condition that has been reported in North America whales but not in Europe.

The disease spreads to heart, brain and lungs if it is infected on the skin of weak animals. This explained why the whale was acting disorientated, according to the prefecture. In this instance, the prefecture said that the disease appeared to be very advanced and likely caused major pain for the whale.

“In such conditions, the experts group determined unanimously that euthanasia was the best option to alleviate suffering as well as to perform advanced analysis on the disease the animal is carrying,” said the prefecture.

The failed rescue operation involved experts from both the French Office for Biodiversity (OFB), and the Sea Shepherd NGO.