The mother of Columbine High School shooter Dylan Klebold says she didn't know anything was wrong with her son before the 1999 attack, and that she thinks about the victims and their families every day.

In an interview set to air on "20/20" on Friday night, Sue Klebold tells ABC News' Diane Sawyer that before the attack she considered herself a parent who would have known something was wrong, but "I didn't know, and it's very hard to live with that."

Dylan Klebold and Eric Harris opened fire at the suburban Denver school on April 20, 1999, killing 12 students and a teacher before killing themselves.

Sue Klebold's interview coincides with the release of her memoir, "A Mother's Reckoning: Living in the Aftermath of Tragedy."