A 46-year-old man who killed a police officer in a gunfight earlier this month committed the crime due to delusional disorder, police said Friday, wrapping up their investigation.

On Oct. 19, Seong Byung-dae killed officer Kim Chang-ho, 54, with a crudely made gun during a shoot-out with the police in front of a tunnel in northern Seoul.

Kim was on a duty after receiving a report that Seong was assaulting his neighbor, surnamed Lee, according to police.

When apprehended on the same day, Seong was carrying six guns, all made of wood, police said.

After reviewing Seong's medical records and statements, police concluded that he has been paranoid that police and prison staff will harm him ever since he served over nine years of jail time for sexual assault charges.

"Isolation from society made him harbor distorted anger towards an unspecified mass and led him to manufacture the guns and attack the officer," police said. "Based on the anger he had developed with his paranoia and delusions, he thoroughly planned the crime."

While serving the prison term, he had a record of getting treatment for schizophrenia four times, police said.

"Records show that he did not take the pills related to the mental illness he was prescribed," police said. "There is no record of him getting treatment since he was released." (Yonhap)