Today marks the 11th anniversary of the Columbine massacre. On April 20, 1999, 17-year-old Dylan Klebold and 18-year-old Eric Harris killed 12 students and one teacher in the deadliest American high-school shooting spree of all time.

Susan Klebold, Dylan's mother, has never given a public interview, but she wrote an eight-page essay in Oprah's O magazine last spring, detailing her disbelief and anguish.

"For the rest of my life, I will be haunted by the horror and anguish Dylan caused," she wrote. "I cannot look at a child in a grocery store or on the street without thinking about how my son's schoolmates spent the last moments of their lives. Dylan changed everything I believed about myself, about God, about family and about love."

Susan admitted that she had no idea her son was in any emotional turmoil. Experts who've analyzed Dylan's writings concluded that he was depressed and suicidal. "I'd had no inkling of the battle Dylan was waging in his mind," Susan wrote.

It's more important now than ever to be able to spot the warning signs of suicidal thoughts and depression in our teens -- so we can take action before it's too late.

Our hearts go out to the families of the vicitims of the Columbine tragedy.