Photos of what might be a dead juvenile blue whale have recently surfaced online, allegedly showing the catch of Icelandic whalers.
It's a species that the International Union for the Conservation of Nature (IUCN) says hasn't been deliberately killed since 1978.
If confirmed, this will cause serious problems for the whalers in question, as it's illegal to deliberately kill blue whales in Iceland, and it highlights a troubling whaling practice.
"While I can't entirely rule out the possibility that this is a hybrid, I don't see any characteristics that would suggest that," Phillip Clapham, from the US National Oceanic and Atmospheric Administration (NOAA) Alaska Fisheries Science Centre, said in a Sea Shepherd statement.
"From the photos, it has all the characteristics of a blue whale; given that - notably the coloration pattern – there is almost no possibility that an experienced observer would have misidentified it as anything else at sea."
The company that killed the whale, called Hvalur hf, has other ideas though, saying it's sure the whale is a hybrid.
"I am absolutely confident that it's a hybrid," said Kristján Loftsson who runs Hvalur hf, to Matt McGrath at the BBC.
"To mistake a blue whale for a fin whale is impossible, this whale has all the characterisations of a fin whale in the ocean. There are a lot of blue whales off the Iceland coast, when we see the blows and sail to it, and we realise it is a blue and then we leave it and go and look for fin whales."
Here's why this 'hybrid' or 'not hybrid' distinction is so important.
Iceland, along with every other country, agreed to honour the International Whaling Commission's decision in the 1960s. It makes hunting for blue whales illegal, and as far as we know, none have been deliberately killed since 1978.
However, Iceland doesn't follow the international moratorium on killing all whales, and will give permits for hunting fin whales, which they do not believe are threatened.
And, according to the Whale and Dolphin Conservation, Icelandic whalers have killed four hybrid blue and fin whales over the years.
Although they are quite rare, a hybrid whale is a legal grey area, giving the whalers the option to say that they made a mistake.
Campaign group Hard to Port published the photo and documented the killing on Monday via Facebook.
"Whale 22 (documented by us on July 7th midnight / July 8th early morning) shows features of a blue whale (darker belly, all black baleen, bluish colour)," they wrote in a Facebook post.
"Hard To Port also noticed the critical examination by Kristján Loftsson and his staff of this particular animal."
The government has promised an investigation and DNA test to determine what this animal is, but campaigners are, in fact, hoping that the damage has already been done.
"We hope it might be the nail in the coffin of Icelandic whaling," Astrid Fuchs from the Whale and Dolphin Conservation told the BBC.
"It confirms what scientists have been saying for years, whaling can't be regulated - it is always a bit out of control, they are going out there but they don't know what they are shooting.
"If this is a blue it would drive home the message that you can't regulate this."