The Plymouth gunman was not charged or cautioned after he carried out a vicious assault in a city park last year, but instead was allowed to take part in a four-month rehabilitation scheme, it has emerged.

Jack Davison’s legally held shotgun was confiscated after he began the Pathfinder scheme and a worker from it found out he had the weapon and informed Devon and Cornwall police. But the shotgun was handed back last month after he finished the programme.

The police watchdog, the Independent Office for Police Conduct (IOPC), said it was examining the force’s decision to “divert Davison from prosecution for the assaults” as well as why he was given the weapon back.

Davison, 22, shot dead five people including his mother, Maxine, 51, after a row at their home in Keyham last Thursday. After killing his mother, Davison left the house and shot dead Lee Martyn, 43, and his daughter Sophie Martyn, three, Stephen Washington, 59, and Kate Shepherd, 66. Davison, an apprentice crane operator, then turned the shotgun on himself.

His social media accounts have revealed a hatred for women, homophobia and a deep interest in the “incel” or “involuntary celibate” movement. The IOPC is to examine whether anyone looked at Davison’s social media accounts before he was issued with his shotgun certificate.

It is also examining whether the force had any information from Davison’s GP and any other medical or mental health services concerning his state of mind.

The Guardian can reveal that on his now deleted Reddit account, Davison said he went to a GP two weeks before the attack seeking treatment for “mood problems” after taking high-strength steroids during lockdown.

He complained of having “super high levels of prolactin”, which can lead to depression, impotence and weight gain. On 22 July, Davison said his health was “making me panic”. After having trouble booking an appointment, he wrote that he was due to see a doctor about his “libido issues” and “mood problems” on 28 July. After the appointment, he wrote in a subreddit called Virgin that he was disturbed by a male doctor touching him as part of the examination.

In June, in a subreddit called Testosterone, he said he was going to the doctor as soon as possible after admitting to “stupidly” taking a potent steroid during lockdown. In early June he complained of “tons of other physical mental sexual issues” and described himself as “dogmatic, insecure” and in an “overall bad mood”.

The IOPC revealed that Davison applied for a shotgun certificate in July 2017, when he was 18, and a certificate was issued to him by police in January 2018, valid for five years. He is believed to have legally bought a shotgun in March 2018.

In September 2020, Davison assaulted two youths in a park in Plymouth. He was identified as the suspect the following month and admitted the attack.

It was decided that the offences fitted the criteria for the force’s Pathfinder scheme. According to the IOPC, a scheme worker raised concerns with the force’s firearms licensing department at the end of November 2020 that Davison had a shotgun, and the weapon and certificate were seized by police on 7 December 2020.

Davison completed the Pathfinder scheme in March 2021 and, after a review by the firearms licensing department, the shotgun and certificate were returned to him on 9 July.

The IOPC is investigating whether relevant laws, policies and procedures were followed. It is also examining any sharing of information between the firearms licensing section and the part of the force that was aware that Davison had been identified as a suspect for assault.