In the early morning of Tuesday July 26, a lone attacker with a backpack full of knives stabbed 19 people to death and injured at least 25 more at a care home for disabled people in the city of Sagamihara, about thirty kilometers west of central Tokyo. The assailant was arrested and identified as 26-year-old Satoshi Uematsu, a former employee of the care home. The attack is Japan’s worst case of mass murder since the end of WWII.

As more details of the crime emerged, Uematsu was quoted as saying he held a grudge against the care home after he had been fired from there in 2012. After turning himself in to police, the Asahi Shimbun reported that Uematsu said, “It would be better if disabled people were dead.” Uematsu had also attempted to deliver a letter to a local legislator in February stating that disabled people should be poisoned. He was committed to a psychiatric hospital after this act (link in Japanese), and even though a physician reported he was at risk of killing others, he was released a few days later anyway.

“Rampage killings” such as the Sagamihara attack, where one assailant kills a large number of strangers, have been rare in post-war Japan. While rampage killings have become more frequent over the past decade in the US, there have been just three mass killings in Japan in the past fifteen years. In June 2008, Tomohiro Kato killed seven and injured ten in the Tokyo entertainment district of Akihabara after driving into a crowd with a truck and then attacking bystanders with a knife. In June 2001, Mamoru Takuma stabbed eight students to death and wounded 15 others at an elementary school in Osaka. And now Sagamihara.

Since 1945, these kinds of massacres seem to occur about once a decade in Japan, and other horrible crimes are reported fairly regularly. But compared to other countries, Japan has a very low murder rate, period. So why are these attacks so rare in Japan compared to other countries?

According to the latest Organization for Economic Cooperation and Development (OECD) data, Japan’s homicide rate was just 0.3 per 100,000 people in 2015. This is the second-lowest rate in the OECD, where the average homicide rate is 4.1 per 100,000. Annual statistics compiled by Japan’s National Policy Agency show there was only one gun-related death in 2015 in Japan (link in Japanese), and six in 2014. In contrast, at the other end of the spectrum, there were between 11,000 and 12,000 homicides with guns in the US in 2014.

What reasons are behind Japan’s low homicide rate, especially those murders involving guns? For one thing, gun ownership in Japan is very rare. There are just 0.6 firearms per 100 people in Japan, compared to 88.8 in the US. But as the Sagamihara, Akihabara and Osaka massacres demonstrate, if an assailant wants to kill people, they do not need a gun to do so. But if there were 372 mass shootings in the US in 2015, why didn’t Japan have a similar number of knife massacres that year?

Next-to-nonexistent household gun ownership aside, it could also be argued that Japan’s comparatively communal culture—which places a strong emphasis on “preserving honor,” “avoiding shame,” and generally being honest—may reduce petty crime as well as keep murder rates low. Besides murder, rates of assault and public disorder are much lower in Japan than in other parts of the world. Culturally, getting angry in public in Japan is considered a major faux pas, and only something that children do. The Japanese education system, with its emphasis on collaboration and not just on competition, could be one more reason why the murder rates are so low.

Another explanation may be that drug use in Japan is almost nonexistent and is frowned upon socially. Without the pervasive presence of narcotics, the general mayhem and crime associated with drugs in many other countries is not present. And then there’s also the idea that Japanese society simply rejected violence as a way of approaching problems following the country’s crushing defeat in WWII.

For whatever unquantifiable reason you want to put it down to, the end result is that murder rates are incredibly low in Japan. In that case, what drives young men to commit massacres?

Little is known yet of Satoshi Uematsu, the 26-year-old who committed today’s attacks in Sagamihara. He was a young man who became socially isolated after losing what was a fairly low-status job as a caregiver in 2012. In February 2016, he was diagnosed with an as-yet-unspecified mental illness before being released from a psychiatric hospital. Six months later, he killed at least 19 people.

Mamoru Takuma, who stabbed eight children to death and wounded 15 others at an elementary school in Osaka in 2001, was also living with untreated mental illness, and was also socially isolated and estranged from his family. Tomohiro Kato, who killed 7 and injured 10 in Akihabara in 2008, wasn’t living with a diagnosed mental health condition. But he was likewise estranged from his parents and any support network and living a precarious existence as contract employee, deeply in debt.

The common theme? Socially isolated men, often living with untreated mental illness or the effects of childhood emotional or physical abuse, acting out. While Japan may be a safe country compared to many other parts of the world, it’s not always safe for everyone, all the time. Recognizing this fact can help the country take steps toward making citizens even safer.