The man arrested Friday for a rampage that injured 14 people outside JR Toride Station in Ibaraki Prefecture allegedly told police he bought the knife used in the incident about a year ago, around the time he quit his last job.
Police sources said Saturday that Yuta Saito, 27, could have harbored criminal intent for about a year, since around the time he quit working after a period of hopping between jobs and bought the 25-cm-long kitchen knife at a low-price retail shop.
The police have turned Saito over to prosecutors on suspicion of attempted murder.
He is believed to have kept the knife at his home along with a 20-cm fruit knife that he also had with him Friday, the police said.
Investigators believe Saito stayed away from home for two nights until Friday.
The rampage in front of the station left 12 teenage students and two women injured.