Baghdad, Iraq ? 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,” national security adviser Mouwafak al-Rubaie said. “Our troops have dealt fatal and painful blows to this organization.”

He accused the terror suspect 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 on June 7 north of Baghdad.

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, 60 miles 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 U.S.-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 Sunday.

The U.S. military command announced that four U.S. 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.

As of Sunday, at least 2,647 members of the U.S. military have died since the beginning of the Iraq war in March 2003, according to an Associated Press count.