Sue Klebold, whose son Dylan, 17, went on a shooting rampage with his friend Eric Harris, 18, at their high school in Colorado in 1999, says she still thinks about the victims every day.

Klebold said she still loves her son and does not believe he was a monster but cannot forgive herself for not realising something was wrong.

Klebold, 66, also admits that while she was shocked and devastated by her son’s role, she did not feel anger until six months after the killings, when investigators showed her the home video tapes Dylan and Harris had made.

Klebold still cannot believe she failed to add up the signs that Dylan was depressed to the point of being suicidal.

Klebold’s book, A Mother’s Reckoning: Living in the Aftermath of the Columbine Tragedy, is to be published on 15 February, with all proceeds being donated to charitable foundations that focus on mental health issues.

In the book, she pieces together every memory she has of Dylan’s childhood, but still finds it almost impossible to match the child she called her “Sunshine boy” with the merciless sadist he became, with an expression of “Sneering superiority” she had never seen before.

None of the killers’ parents spoke out publicly for 10 years, then in 2009 Klebold wrote a first-person article for Oprah Winfrey’s magazine, O. Now, another seven years on, she goes further, to talk and write in detail about coming to terms with Dylan’s willing participation in the planning of the murders.

Despite her son’s appalling place in history, Klebold still cannot but love him.

Read the full exclusive interview with Sue Klebold on from Sunday, and in Guardian Weekend magazine on Saturday.

Dylan Klebold