Queensland authorities are investigating a serial croc killer after an anonymous caller claimed responsibility for two decapitated crocodiles found in the state’s far north in the last two months. A local crocodile tour operator, David White, received the call from a mystery man shortly after discovering the decapitated head and spine of Lizzie, a 40-year-old female saltwater crocodile that he had been observing for more than two decades, on the banks of the Daintree River in May. “He said, ‘I am the apex predator now, I killed a crocodile,’” White said. “She was my favourite female crocodile, the first one I named out there. I named her after Queen Elizabeth because she had a habit of waving at you like the queen.” White said the man claimed he had decapitated Lizzie and a much larger crocodile found rotting in Cow Bay in April. In the Cow Bay case, the crocodile’s head was taken and the body was left behind. Although the cause of Lizzie’s death is still under investigation, White said it was obvious from the way her head and spine had been cleanly removed that it was at the hand of humans. On Friday, a spokesperson for the Queensland environment department, said authorities were investigating links between the two discoveries and said they feared the protected animals were being “targeted and deliberately killed”. Saltwater crocodiles were hunted to the brink of extinction in the early 1970s until conservation efforts allowed populations to rebound, resulting in a gradual increase in attacks on humans. Sign up for Guardian Australia’s free morning and afternoon email newsletters for your daily news roundup There are now an estimated 20,000 to 30,000 crocodiles in Queensland, prompting the Katter Australia party to call for a cull of the species, though crocodiles are still listed as vulnerable. Since 1985 there have been 42 crocodile attacks in Queensland – 12 of them fatal. The KAP state leader, Robbie Katter, said his party was calling for some control of the numbers because of the population explosion. “The problem we have is regardless of whether you want to debate the numbers or not, we are seeing crocs in places they haven’t been before,” he said. The issue has resurfaced after the discovery of fisher Kevin Darmody’s remains inside two crocodiles that were shot by authorities near his capsized boat far north Queensland in May. There are now fears of vigilante attacks. “No one is doing anything,” said a north Queensland crocodile photographer and conservationist, Tom Hayes. “When I tried to tip off police, they sounded like they were annoyed that I was even talking about it … “I am not a conspiracy theorist or anything, but why does there seem to be cover-ups in the world of crocodiles?” He added: “I’ve been doing some digging and I’ve narrowed it down to about four people that I’m pretty sure were involved in the Cow Bay killing.” Hayes said he was only a “matter of days” from closing in on the culprit but was not fully convinced they were responsible for the death of Lizzie as well. “If you’re going to kill a crocodile and take the head, why would you kill another crocodile and not take the head?” he said. Hayes said it was likely that the call could have been a sadistic attempt to ridicule White. White has also considered this theory, as he has often been subject to online bullying for his attachment to the predators. “They can’t understand how people can get attached to a crocodile,” he said. “But for me, it’s just like every member of my family. It’s just a real tragedy.” A crocodile researcher, Mick “Mullet” Bender, believes the government has a responsibility to do more, particularly regarding education, and said he believed this the issue was actually bringing the community together. “Despite their views on whether they think crocodiles need culling or not, they still think that what that guy’s done is terrible, because he’s gone and killed a female crocodile in a tourism area, where the locals rely on the crocs for their income,” Bender said. “This feels more like a direct attack at the crocodile conservation community.” Anyone with information about these killings or about people in possession of crocodile skulls or other body parts should call 1300 130 372 or contact the Queensland police service. The maximum penalty for the deliberate harm or killing of crocodiles in Queensland is $32,243.