( CNN ) -- A Washington state middle school boy was arrested after he brought 400 rounds of ammunition , multiple knives and a handgun to his school , intending to shoot another student who he thought was bullying his friend , police said . The 11-year-old was booked into a juvenile detention facility Wednesday after the incident that caused the lockdown of Frontier Middle School , Vancouver Police said . No one was injured . Authorities were alerted by the boy 's mother , who called the school concerned that her son may have taken kitchen knives with him that day , according to a probable cause document . The boy was patted down by school resource officers who found the weapons and ammunition . The boy told school officials that `` a voice in his head was telling him to kill '' an 11-year-old classmate for calling the boy 's friend gay , according to the probable cause document . `` He stated a voice told him killing ( the classmate ) was a good idea ; however he planned to just shoot ( the classmate ) in the arm and then shoot himself in the head , '' the document said . The boy is being held on charges of attempted assault , unlawful possession of a firearm and possession of a weapon at school . He earlier faced a charge of attempted murder , but prosecutors did not immediately find sufficient evidence to support the charge . However , Kasey Vu with the Clark County Prosecuting Attorney 's Office left open the possibility that the attempted murder charge could return when prosecutors receive more information from the investigation . The school in Vancouver was locked down for about two hours . Students were sent home with letters alerting parents of the situation , said Kris Fay , a spokeswoman for Evergreen Public Schools . The incident comes on the heels of several school-related violent incidents around the nation . On Monday , a math teacher was killed and two students wounded by a boy who then killed himself in a Nevada school . On Wednesday , a math teacher was found dead in the woods northeast of Boston ; her 14-year-old student is accused of killing her . Three California students injured after gun on police motorcycle discharges . Teacher slayings hurt view of schools as safe havens . CNN 's Greg Morrison , Drew Iden and Tina Burnside contributed to this report .
NEW : 11-year - old said a voice in his head told him to hit a classmate , the document says . Incident occurred in Seattle , Washington . No one was injured . It came on the heels of several school - related violent incidents .