A mother has admitted helping her bullied teenage son build a cache of weapons he had planned to use in a school massacre in the United States.

Michele Cossey, 46, pleaded guilty in a Pennsylvania court to one count of child endangerment.

She admitted she had bought her 14-year-old son Dillon - who left school at the age of 12 because he was bullied about his weight - a rifle with a laser scope and gunpowder, which investigators said he was using to build grenades.

Prosecutors said Dillon idolised the Columbine High School shooters and was planning an attack last year on Plymouth Whitemarsh High School in Pennsylvania, which some former schoolmates attended.

After leaving school because of the harassment he suffered, Cossey was home-schooled.

Montgomery County Assistant District Attorney Christopher Parisi said the Michele Cossey 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".