The U.S. Defense Intelligence Agency has attempted to block a book about the tipping point in Afghanistan and a controversial pre-9/11 data mining project called Able Danger." In a letter obtained by Fox News, the DIA says national security could be breached if Operation Dark Heart is published in its current form. The agency also attempted to block key portions of the book that claim "Able Danger" successfully identified hijacker Mohammed Atta as a threat to the United States before the Sept. 11, 2001, terror attacks. In a highly unusual move, the Department of Defense is now negotiating with the publisher, St. Martin's Press, to buy all 10,000 copies of the first printing of the book to keep it off shelves -- even after the U.S. Army had cleared the book for release. Specifically, the DIA wanted references to a meeting between Lt. Col. Tony Shaffer, the book's author, and the executive director of the 9/11 Commission, Philip Zelikow, removed. In that meeting, which took place in Afghanistan, Shaffer alleges the commission was told about "Able Danger" and the identification of Atta before the attacks. No mention of this was made in the final 9/11 report. Once back in the U.S., Shaffer says he contacted the commission. Without explanation, the commission was no longer interested.