Marine biologists and whale experts have stepped up their criticism of a privately funded operation to release a humpback whale that was stranded for weeks off Germany’s Baltic coast after it emerged that a tracker fitted to the whale was not working.

The whereabouts and health of the young male whale – nicknamed Timmy after one of the sandbanks it was stranded on – remain unknown three days after it was transported in a water-holding barge pulled by a tugboat to waters off the coast of Denmark.

The rescue initiative, estimated to have cost about €1.5m, was funded in part by Karin Walter-Mommert, the owner of one of the largest racehorse portfolios in Europe.

Walter-Mommert, who confirmed to German media on Tuesday that the tracker was not working, had previously claimed the device was also supposed to transmit information on the animal’s vital signs. This was disputed by some experts and environmental activists, including Greenpeace.

The whale was first spotted stuck on a sandbank on 23 March near the city of Lübeck, on Germany’s Baltic Sea coast, before freeing itself and then becoming stuck again several times.

The mammal’s health deteriorated as it became repeatedly stranded in shallow waters near the coastal city of Wismar, and unsuccessful efforts to coax it toward deeper seas were livestreamed across the globe.

The environment minister for Germany’s Mecklenburg-Western Pomerania state gave the green light for the attempt to save the whale, despite some warnings from the scientific community that it may be too much for the animal.

“If it turns out that the device doesn’t yield any information, it would be an all-round catastrophe, for the whale and the rescue team,” whale researcher Fabian Ritter told German media. Ritter said that if it was not possible to determine if the whale had died then the entire operation would have been in vain.

In a statement the German Oceanographic Museum in Stralsund said it believed the whale was “highly likely” dead and called on organisers of the rescue mission to release data from the tracker so that the operation could be assessed.

Its scientists had been sceptical of the attempts to rescue the whale from the start, referring to it as animal cruelty, and stressing the likelihood that the whale would drown if it was not strong enough to swim in deep waters.

The whale was last photographed swimming in the strait of Skagerrak. Danish authorities said in advance they would make no attempt to rescue the whale if it was subsequently found to be in difficulty, arguing that nature should be allowed to take its course.

Danish marine biologist Peter Madsen said that the lack of data from all stages of the operation was unusual and ill-advised.

He told the German Press Agency (DPA) that it appeared data was being guarded by a small group of people, including the initiators of the operation and the environment ministry for the state of Mecklenburg-Vorpommern, calling it “very strange and unprofessional”. The environment ministry said it was still waiting to receive data from the private initiative, as well as information about the animal’s condition.

Neither the rescue initiative nor the environment ministry responded to requests for information about which tracking device was being used.

There were also confusing reports surrounding the decision to release the whale. Kirsten Tönnies, a vet who had been on board the Fortuna B, one of two rescue ships accompanying the whale, was reportedly barred from witnessing the second and final release attempts.

Tönnies said that tensions had been high between the experts on board and the ship’s crew. She said she disagreed with how the whale was released backward from the barge and with the fact she had been barred from giving the medical all-clear beforehand.

The ship’s operators were unavailable for comment. The Fortuna B’s tracking signal appears to have been switched off, according to authorities and vessel trackers.

In a joint statement, Walter-Mommert and her co-financier, Walter Gunz, one of the founders of a leading electronics chain, distanced themselves from the manner of the whale’s release.

“We hereby expressly distance ourselves from the events and the manner in which the whale was abandoned,” they wrote, calling for “any consequences” to be borne “by the owner, the operators, and any crew members of the ships Fortuna B and Robin Hood”.