The bombing of the Al-Askari shrine in February in Samarra triggered a major escalation of sectarian violence in the country.
A National Security Adviser, Muwaffaq al-Rubay'i, identified the suspect as Yousri Fakher Muhammad Ali, a Tunisian.
He said the suspect was seriously wounded during his capture, which happened several days ago near Baghdad.
Al-Rubaie said the mastermind of the bombing, an Iraqi, is still at large.
(compiled from agency reports)
Reactions To The Samarra Attack
Iraqi religious and government leaders, as well as international officials, condemned the February 22 bomb attack that wrecked the Golden Mosque, a major Shi'ite Muslim shrine in Samarra. Below is a selection of statements on the incident.
"This new ugly crime comes as a warning that there is a conspiracy against the Iraqi people to spark a war among brothers. God willing, we will not allow this.... We must cooperate and work together against this danger, the danger of civil war. This is the fiercest danger because it threatens our unity and our country with a devastating civil war." -- Iraqi President Jalal Talabani
"The timing of this crime indicates that one of its aims is to stall the political process and to hamper the negotiations on the formation of a national-unity government." -- President Talabani
"I announce on this occasion three days of mourning. I hope our heroic people will take more care on this occasion to bolster Islamic unity and protect Islamic brotherhood and Iraqi national brotherhood." -- Iraqi Prime Minister Ibrahim al-Ja'fari
"Oh honorable people of Samarra! We should stand as one, united in confronting terrorism.... This assault is an assault on all Muslims." -- Iraqi Interior Minister Bayan Jabur
"They will fail to draw the Iraqi people into civil war as they have failed in the past." -- Iraqi National Security Adviser Muwaffaq al-Rubay'i
"If the security systems are unable to secure necessary protection, the believers are able to do so with the might of God." -- Shi'ite spiritual leader Grand Ayatollah Ali al-Sistani
"We will not only condemn and protest but we will act against those militants. If the Iraqi government does not do its job to defend the Iraqi people we are ready to do so." -- Shi'ite cleric Moqtada al-Sadr, speaking through spokesman Abdel Hadi al-Darajee
Click the image to view RFE/RL's complete coverage of events in Iraq and that country's ongoing transition.