Elizabeth Sturtz, 48, the mother of 17-year-old Shawn Sturtz, said Saturday that bullies often called her 300-pound son, who had a learning disability, fat and stupid, so she understands how her son could have been angry.
Her comments came as authorities announced the arrest of a third teen.
Sturtz said despite the taunting she saw nothing in her son's past or recent behavior showing he would plot a Columbine-like attack on the school.
"They were pushed to the limit. They couldn't handle it anymore," she told The Associated Press in an interview in her home.
Police arrested Sturtz and William Cornell, also 17, on Thursday, and Bradley P. Netwal, 18, on Friday, on suspicion of conspiracy to commit homicide and arson. District Attorney John Zakowski said he planned to file charges against them Thursday.
Investigators said the teens planned to set off bombs near bathrooms, light exits on fire and shoot people who they didn't like at East High School.
Elizabeth Sturtz said Cornell, like her son, weighed 300 pounds and both were subject to bullying at least in part because of their weight. She blamed herself for not doing more to help her son.
"I don't know what to do to help him," she said.
Cornell's lawyer, Shane Brabazon, did not immediately return telephone messages left at his home and office Saturday evening.
Police found out about the plot from a student who alerted an associate principal. A detective said Friday that one boy wanted to carry out the plan Thursday but the other talked him out of it.
Investigators said Netwal was a former student at the school and a friend who participated in the planning. All three were being held in the Brown County Jail.