LONDON — Britain’s police watchdog says it has launched an investigation into why a 22-year-old man who fatally shot five people in southwestern England on Thursday was given back his confiscated gun and gun license last month.

Police have said Jake Davison killed his mother and four other people, including a 3-year-old girl, before taking his own life in the port city of Plymouth. It was Britain’s first mass shooting in over a decade. Firearm crimes are rare in Britain, which has strict gun control laws and regulations.

The Independent Office for Police Conduct said late Friday it would investigate the Devon and Cornwall police department’s decision-making in relation to Davison’s possession of a shotgun and the license. The watchdog said it was not yet known whether the shotgun returned to Davison was the same one he used in Thursday’s shootings.

Police took away the gun and the certificate in December 2020 following an allegation of assault three months earlier, the watchdog office said. They were returned to Davison last month.

“We will examine what police actions were taken and when, the rationale behind police decision-making and whether relevant law, policy and procedures were followed concerning Mr. Davison’s possession of a shotgun,” the office’s regional director, David Ford, said in a statement.

“The investigation will also consider whether the force had any information concerning Mr. Davison’s mental health and if so, if this information was appropriately considered,” Ford said.

Hundreds attended a candlelit vigil in Plymouth Friday, close to where the killings took place.

Police said Friday the motive for the shootings was unclear but there were no immediate signs they were an act of terrorism or that Davison had connections to extremist groups.

They said Davison shot and killed his 51-year-old mother, Maxine Davison, also known as Maxine Chapman, at a house before going into the street and killing 3-year-old Sophie Martyn and her father, Lee Martyn, 43.

According to the police timeline, Davison next killed Stephen Washington, 59, in a nearby park, before fatally shooting Kate Shepherd, 66, on a nearby street.

Two other people were wounded.

Shaun Sawyer, chief constable for Devon and Cornwall police, told reporters that investigators think the crimes started as “domestically related” and “spilled into the street.” He said the investigators were keeping open minds but do not think extremist ideology prompted the attack.

“Let’s see what’s on his hard drive, let’s see what’s on his computer, let’s see what’s on social media,” Sawyer said.

Davison appeared to post on YouTube under the name Professor Waffle. The account has been taken down and replaced with a notice saying it violated the site’s community guidelines. In a final 11-minute clip posted before the killings, “Professor Waffle” talks about how he was “beaten down and defeated by…life.”

He talks about struggling to stay motivated to lose weight and work out, and working as a scaffolder at ages 17-18. He hinted at lacking a love life, referring to “people who are incels” — shorthand for “involuntarily celibate.”

The “incel” movement justifies violence against women as revenge for men who are rejected as sexual partners. The online subculture has been linked to deadly attacks in California, Toronto and Florida.

Davison said that while he wouldn’t describe himself as an “incel,” they are “people similar to me, they’ve had nothing but themselves, and then they’ve socially had it tough.”