A pair of teenage girls planned to commit a mass shooting at their Mountain Vista High School last year before killing themselves, according to an arrest affidavit unsealed Friday, which also shows at least one of the two had researched — and possibly taken steps to obtain — guns. The affidavit for Brooke Higgins — who pleaded guilty in the case last month — for the first time provides details into the alleged plot concocted by then-16 year old Higgins and Sienna Johnson.