WASHINGTON (CNN) -- Senior U.S. intelligence officials call a letter from al Qaeda's No. 2 man to its leader in Iraq "chilling" because of how "calm, clear and well argued" it is in urging preparation for a U.S. departure from Iraq.

According to a translation of the 6,300-word letter provided by the U.S. government, Ayman al-Zawahiri predicts "the Americans will exit soon" from Iraq and says "things may develop faster than we imagine."

U.S. leaders have refused to set a timetable for troop withdrawals, saying such a move would embolden insurgents. Military leaders have suggested a reduction in 2006 is possible, depending on the preparedness of Iraqi security forces.

But in the letter, al-Zawahiri is clearly worried that Abu Musab al-Zarqawi, with his televised beheadings of hostages and attacks on Shiites, could lose what he calls a "media battle" for the "hearts and minds" of Muslims...

...The Egyptian-born physician also says the U.S. hunt has taken a toll on his family -- that his "favorite wife," son and daughter have been killed, but that he has fathered a daughter while in hiding and his health is fine...