Following the murder of 19 people at a care facility by a former staff member, experts are trying to determine the motivation behind the attack.

In the leading hypothesis so far, the killer had anger management problems coupled with ill feelings toward people with disabilities.

Satoshi Uematsu, 26, was arrested on Tuesday following the attack that also left another 25 people injured at the facility in Sagamihara, Kanagawa Prefecture, where he once worked.

“He may have problems coping with anger. Most people are able to control themselves, knowing they could face arrest or hurt others, even though they are upset, so they act more rationally,” said Kenji Omata, a criminal psychology professor at Surugadai University.

“There may be something that happened to him recently” that triggered his murder spree, he said.

For example, it is being reported that Uematsu visited the official residence of Lower House Speaker Tadamori Oshima in February, with a letter proposing lawmakers enact legislation that allows for the mercy killing of people with disabilities.

“Since his letter was rejected, he may have carried out the murders to place himself (and his cause) in the spotlight.”

He added, however, that he had choices other than to kill and he appeared to be acting rationally when he was shown on TV being transported by the police after the murders.

“It’s reported that he had recently dyed his hair, and left his job in February. The change in his social status could have caused him to feel anger and frustration,” Oshima said, saying further examination of his relationships, particularly with his family members, is needed to fully make a determination as to why he acted out in the way he did.

While mass murder is not completely unheard of in Japan, data from the Organisation for Economic Co-operation and Development (OECD) comparing per capita deaths by country due to assault in 2012, the latest available year, ranked Japan as one of the four lowest, with a rate of 0.3 percent per 100,000, along with Iceland, Britain and Denmark. The rate had fallen from 0.4 percent in 2009.

The latest statistics from the National Police Agency also show the number of reported cases of violent crime that includes killing has gradually decreased over the past several years, with 7,625 cases in 2010 and dropping to 6,453 in 2014.

However, most mass killings are often committed by loners who have difficulty socializing with others.

In one such case in September 2015, in Kumagaya, Saitama Prefecture, a 30 year-old Peruvian man named Vayron Jonathan Nakada Ludena was arrested after entering three houses and stabbing six people to death. Co-workers at the deli food factory where he had been employed said he preferred to keep to himself and rarely interacted with others, even during breaks.

In June 2008, Tomohiro Kato, 25, was arrested for killing of seven people and injuring 10 more in an incident where he drove a rented truck into a crowd of pedestrians in Tokyo’s popular electronics district Akihabara before jumping out and slashing and stabbing people. Kato was a temporary worker at a car repair factory in Shizuoka Prefecture. He was a constant visitor to online bulletin board where he posted numerous messages. During his trial, prosecutors said he had a strong sense of being ignored by society and the bulletin board provided his only escape.

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