The letter attributed to Adnan Rashid was released just days after 16-year-old Malala Yousafzai took the stage at the United Nations, where she delivered an emotional plea for the right to go to school on behalf of all children.
Malala was 15 when gunmen jumped on her school bus and shouted her name, scaring other girls into identifying her, in the Swat Valley on October 9, 2012. The attack sparked massive protests in Pakistan and condemnation worldwide.
"The Taliban believe you were intentionally writing against them and running a smear campaign to malign their effort to establish an Islamic system in (the) Swat Valley, and your writings were provocative," according to the letter, which was dated Monday and released to CNN by a Pakistan intelligence source.
"You have said in your speech ... that the pen is mightier than the sword. So they attacked you for your sword not your books or school."