why mammograms fail to detect tumors in some women. cbs roxana saberi has the questions every woman should ask their doctor. this is the cbs evening news with norah o donnell, reporting from the nation s capital. o donnell: good evening to our viewers in the west, and thank you for joining us. we re starting tonight with horror in the heartland of america. another school shooting, this time in st. louis. it happened today at the city s central visual and performing arts high school. terrified students leaving their school with hands raised, escaping the senseless horror of bullets and bloodshed in their hallways and classrooms. a teenaged girl was pronounced dead inside the school. another victim, a teacher, died at the hospital. the gunman, believed to be in his 20s was killed in a gun battle with police. this was the 40th school shooting in the u.s. this year. that s a half dozen more than all of last year. the mayor of st. louis spoke for so many parents today saying