You know, investigating can be difficult. There are many pitfalls, and it is easy to fool yourself, even when others are not trying to fool you. Critical thinking skills are indispensable to any investigative endeavor – along with specific domain knowledge.

Case in point – the mysterious case of the Croydon Cat Killer. For three years the mystery mutilator killed pet cats, eviscerated them, removed their heads and tails, and deposited the remains on display. Tabloids warned that the perpetrator would likely soon move onto humans. Veterinary pathologists and the metropolitan police were flummoxed. PETA offered a reward, and detectives from South Norwood Animal Rescue and Liberty (SNARL) found evidence of human involvement.

Investigators even worked up a profile of the likely suspect (40, male, problems with women, etc.).

Recently the metropolitan police announced that they solved the case, and the perpetrator is – normal predation and scavenging.

“In three instances where CCTV was obtained, footage showed foxes carrying bodies or body-parts of cats,” police said, referring to surveillance video. A forensic veterinarian on the task force concluded that “the mutilations had been caused by predation and/or scavenging, and highlighted that fox DNA had been found around the wound sites on all five bodies.”

This is, of course, an epic and embarrassing fail, which is why it is spreading so enthusiastically on social media. What went wrong?

Well, an SGU listener who happens to work for the London Metropolitan Police wrote me the following:

While I was not part of the investigation, when I first read about it back in 2016, I was immediately reminded of cattle mutilations and how those incidents can be explained by natural causes. The associated moral panic, which you have previously talked about on the show, was for me, another interesting angle to the story.

Too bad this skeptic wasn’t on the case, because his immediate reaction turned out to be correct, and the analogy is perfect.

UFO enthusiasts claim that co-called cattle mutilations are evidence that aliens have come to the Earth to experiment on our livestock, leaving the remains behind with evidence of their advanced surgical techniques. There are problems with such claims, however (other than the obvious fact that it is absurd).

It is a good example of what none-other than Sherlock Holmes warned about,

“It is a capital mistake to theorize before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts.”

In the case of cattle mutilations, apparent anomalies were interpreted through the lens of belief in alien visitations. For example, cattle are sometime eviscerated, their tongues and some internal organs removed. Other observers note that cuts were perfectly straight, as if they were made by a laser.

Scavengers, however, are known to eat the tongues and internal organs of dead animals. Also, when a dead animal dehydrates, their hide stretches. The stretching can make any cut seem laser straight. The FBI was even called in to investigate, and concluded that the most credible interpretation was natural predation and scavenging.

With the cat killer, theories ran wild without any supporting evidence. Once there was a full panic in place that some psychopathic killer was practicing on pets, then all the details were made to fit that theory. Cats were killed by some blunt force trauma, then mutilated, then moved. So therefore the killer was bludgeoning the cats, then playing with their bodies, and then leaving them for display.

One investigative failure (also pointed out by Holmes) was a failure to consider alternative explanations. Even when you have a theory that you can make to fit the evidence, you have to ask yourself – are there other potential theories (perhaps simpler ones) that also fit the evidence?

In this case, if someone had seriously asked – could this be simple predation and scavenging, the case could have been closed three years ago. Essentially, foxes were feeding on cat roadkill. Perhaps they were sometimes killing them outright. That’s it. Foxes. I think Occam may also have something to say here.

The meta-lesson here, again, is that critical thinking skills are essential to any investigation (medical, criminal, scientific). There are numerous examples of law enforcement investigations gone horribly wrong because of lack of skepticism.

Professional investigators tend to learn their critical thinking skills on the job, through experience. However, it would be better to supplement this kind of learning with more formal education in the critical thinking of investigations.

When I was in medical school I took an elective course on applying lessons from the stories of Sherlock Holmes to medical diagnosis and investigations. It was probably the best course I took in medical school. Courses like this should not be an elective.

The Croydon Cat Killer case is yet another example that demonstrates the need for more formal training in critical thinking. It will also certainly live on as an infamous case in the skeptical repertoire.