Prosecutors said her son, Dillon, came idolize the Columbine High School shooters and was planning an attack last year on Plymouth Whitemarsh High School, which some former schoolmates attended.
Cossey, bullied over his weight, had left public school in seventh grade and was being home-schooled. Over time, violent Internet sites fueled his revenge fantasies, his defense lawyer said after his juvenile court plea.
Montgomery County Assistant District Attorney Christopher Parisi said he thought purchasing the weapons was "an attempt to boost his self-esteem, and in some way help the child, as misplaced as those thoughts may have been."
Michelle Cossey's sentencing hearing won't happen until after she undergoes a psychiatric evaluation. The maximum possible prison term is 3 1/2 to 7 years, but her defense attorney she could get less than a year -- or even just probation -- under sentencing guidelines.
Parisi said he doesn't know if Cossey knew about her son's attack plans, but that he hopes to learn that before sentencing.
"If it were to come out that she knew he was planning an attack ... that would certainly increase the severity of the crime," he said.
The judge who sentenced Dillon Cossey to a juvenile treatment facility, where he could remain until his 21st birthday, said that Michele Cossey had fostered a "me-and-mom-against-the-world" attitude in her only child.
Authorities did not think the school attack was imminent, but the boy did amass an arsenal -- knives, swords, BB guns, the rifle and partly assembled homemade grenades -- in his bedroom at his Plymouth Township home.
Police learned of the planned attack when Cossey invited a friend to join him. The friend went to police last fall.
Michelle Cossey has had twice-a-month supervised visits with her son, is missing him and wants him back home, defense lawyer Tim Woodward said.
"Her ultimate goal is to be reunited with her son," the attorney said. "She does admit that she made some mistakes."
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