Scene after Baghdad blast, Sept. 2003.  (Photo: AP ) As attacks by militants in Iraq continue, more is being learned about the groups behind the insurgency and their motivations. While some are foreign fighters, such as many of the supporters of the late Jordanian militant Abu Musab al-Zarqawi's al Qaeda in Iraq, many guerrilla leaders at the core of the movement come from various corners of Saddam's Baath Party. Most are fighting for a bigger role in a secular society, not a Taliban-like Islamic state, U.S. officials said. While analysts disagree over the size of the insurgency, new estimates run as high as 20,000 fighters – far larger than the 5,000 previously reported. <<< Click on the names to read about guerrilla and religious extremist groups officials believe have taken shape in Iraq. Credits:  CBS News, The Associated Press