A 15-year-old middle school student is under investigation for allegedly shooting a steel ball at the window ofMetro train passing by his home, police said on Wednesday.

A netizen put up a post on microblog siteweibo.com last Sunday showing a cracked window on a carriage on Metro Line 4.

Police immediately contacted the netizen to start an investigation.

They said the window was hit from the outside as the train was running from Shanghai Railway Station to Baoshan Road Station about 1pm last Sunday.

The impact left a round hole in the outer layer of the window pane, police said.

Police located the suspect on Tuesday afternoon in a residential complex on Qiujiang Road and found two imitationguns for shooting plastic bullets, two catapults and about 100 steel and plastic balls at his home.

Two imitation guns police found in the boy's home.

Pellets police found in the boy's home.

Police said the pellets found in the boy's home matched fragments found at the scene.

They said the teenager had admitted firing a pellet with a catapult from the window of his apartment.

His mother accompanied the boy as he was being questioned.

The boy had beentreated by psychiatrists before the incident, police said.