NEW : One victim 's mother accuses authorities of staging arrests . Mexico City 's prosecutor says 12 kidnapped teens were killed . Comment: The teens ' kidnapping in broad daylight has jolted Mexico 's capital .
NEW : One victim 's mother accuses authorities of staging arrests . Mexico City 's prosecutor says 12 kidnapped teens were killed . The motive was revenge , Mexico City 's prosecutor tells reporters . The teens ' kidnapping in broad daylight has jolted Mexico 's capital .