A teenager in custody for assaulting a Tokyo police officer with a knife while trying to rob him of his handgun had told others of his plan, investigators learned Thursday.

Police sources said those whom the boy confided in on Sunday attempted to dissuade the 15-year-old from going through with his plan. The attack came the following day.

The sources also said investigators have learned that the boy had stolen the bicycle used in the attack on Jan. 17. The boy was reported to have been under severe mental strain since late January, and police are investigating his behavior from the day of the bike theft to the night of the attack, the sources said.

The investigation so far indicates the boy began playing survival games with his friends during his first year in junior high school. Four air guns used in these games were seized from his home in a raid following his arrest.

He was quoted as telling investigators that he wanted to fire a real handgun because it felt good to shoot an air gun, the sources said. But he was further quoted as saying that, in retrospect, he cannot understand why he acted the way he did, the sources said.