Authorities on Sunday announced the capture of al-Qaida in Iraq's No. 2 leader, accusing him of "brutal and merciless" terror operations, including the bombing of a Shiite shrine that touched off the sectarian bloodletting pushing Iraq toward civil war. Iraq's national security adviser said Hamed Jumaa Farid al-Saeedi, known as Abu Humam or Abu Rana, was arrested a few days ago as he hid in a residential building southwest of Baqouba. The arrest has left al-Qaida in Iraq suffering a "serious leadership crisis," Mouwafak al-Rubaie said. "Our troops have dealt fatal and painful blows to this organization." He accused al-Saeedi of supervising the creation of death squads and ordering assassinations, bombings, kidnappings and attacks on Iraqi police and army checkpoints. "The operations were brutal and merciless," al-Rubaie said. Not much is known about al-Saeedi, but al-Rubaie said he was the second most important al-Qaida in Iraq leader after Abu Ayyub al-Masri. Al-Masri is believed to have taken over the group after a U.S. air strike killed leader Abu Musab al-Zarqawi north of Baghdad on June 7. Al-Rubaie said al-Saeedi was "directly responsible" for Haitham Sabah Shaker Mohammed al-Badri, an Iraqi whom authorities have accused of leading the Feb. 22 bombing against the Shiite shrine in Samarra, 100 kilometers north of Baghdad. The attack inflamed tensions between Shiite and Sunni Muslims and triggered reprisal attacks that have killed hundreds of Iraqis. Al-Saeedi's capture "will affect al-Qaida in Iraq and its operations against our people, especially those aimed at inciting sectarian strife," al-Rubaie said. The US-led coalition and Iraqi authorities have announced numerous arrests after al-Zarqawi was killed that officials claim have thrown al-Qaida in Iraq into disarray. But rampant sectarian violence and other attacks have continued, with at least 20 Iraqis killed in bomb attacks and shootings on Sunday. The US military command announced that four US troops had been killed - two soldiers killed by a roadside bomb Sunday in Baghdad and two Marines in separate incidents Friday and Sunday in the volatile Anbar province, west of the capital. The arrest of al-Saeedi came just over two months after Iraqi authorities announced that they had captured Yousri Fakher Mohammed Ali, a Tunisian also known as Abu Qudama. "This is a very important development," Deputy Prime Minister Barham Saleh said on CNN's "Late Edition." "Deliberate intelligence work, both by Iraqi forces as well as the multinational forces, have dealt a very severe blow to al-Qaida organization in Iraq," Saleh said. "It is also significant because this man is believed to have been responsible for the attack on the shrines in Samarra, which led to the sectarian violence that we have seen." A senior coalition official told The Associated Press that coalition forces were involved in al-Saeedi's arrest, but would not give details on what role they played. The official, speaking on condition of anonymity because announcements were being made by Iraqi authorities, said al-Saeedi had been arrested along with three other people near Baqouba, 35 miles northeast of Baghdad. Al-Zarqawi was killed on the outskirts of Baqouba. Al-Saeedi "claims to be responsible for more attacks than he can remember" and has been involved in the insurgency almost from its beginning three years ago, the official said. Al-Rubaie said al-Saeedi gave information that led to the capture or death of 11 other top al-Qaida in Iraq figures and nine lower-level members. He said those arrested included non-Iraqi Arabs, but would not give any further information for security reasons. The arrest "will affect al-Qaida in Iraq and its operations against our people, especially those aimed at inciting sectarian strife," al-Rubaie said.