Michele Cossey, 46, pleaded guilty on Tuesday to one count of child endangerment. She admitted that she bought him a rifle with a laser scope and gunpowder, which investigators said he was using to build grenades.

Prosecutors said her son, Dillon, came to idolise the Columbine High School shooters, who killed 12 classmates and a teacher before committing suicide in 1999 in Colorado, 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 half way through middle school and was being home-schooled. Over time, violent Internet sites fueled his revenge fantasies, his defence lawyer said after his juvenile court plea.

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 to 7 years, but her defense attorney she could get less than a year - or even just probation - under sentencing guidelines.

Authorities did not think the school attack was imminent, but Dillon Cossey did amass an arsenal - knives, swords, BB guns, the rifle and partly assembled homemade grenades - in his bedroom at his home in Plymouth Township, about 30km northwest of Philadelphia.

Police learned of the planned attack when Cossey invited a friend to join him. The friend went to police last fall. Dillon Cossey was sentenced to a juvenile treatment facility. -- AP