Mena says the stepfather of one of the boys told police that his handguns had been stolen from his gun locker.
Investigators believe his 16-year-old stepson pried open the gun locker about a month ago and gave the firearms to a 15-year-old friend.
Mena says the two boys admitted they took the guns to school three times and planned to start shooting during a school assembly. Police say the suspects had not decided what day to attack and did not have specific targets.
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