Al Qaeda operatives based in Iran worked on chemical and biological weapons, according to a letter written to Osama bin Laden that is described in a new book by a top former U.S. intelligence official.

The letter was captured by a U.S. military sensitive site exploitation team during the raid on bin Laden’s Abbottabad headquarters in May 2011. It is described in Field of Fight, out Tuesday from Lieutenant General Michael Flynn, the former head of the Defense Intelligence Agency, and Michael Ledeen of the Foundation for Defense of Democracies.