Iraqi and U.S. forces detained a top aide to radical Shi'ite cleric Moqtada al-Sadr in a raid in eastern Baghdad Friday.
Al-Sadr's office said the man arrested is its media director, Sheik Abdul Hadi al-Darraji.
A U.S. military statement did not identify the detainee, but described him as a leader of an illegal armed group involved in the kidnapping, torture and murder of Iraqi civilians.
The deputy coalition commander in Iraq, British Lieutenant General Graeme Lamb, said he is sure the coalition arrested the right man based upon the evidence and intelligence it had.
Iraqi Prime Minister Nouri al-Maliki is under pressure to crack down on Shi'ite militiamen, including those connected to his ally, Moqtada al-Sadr.
The U.S. military also said a roadside bomb blast in Baghdad killed one American soldier Thursday.
In other news, the military announced Friday that Iraqi forces captured the suspected leader of several al-Qaida in Iraq terrorist cells during raids in the Samarra region this week. It said all of the raids were Iraqi operations with U.S. forces providing support.
And, Iraqi forces shot and killed one insurgent after his suicide vest failed to detonate near a checkpoint in a suburb of Ramadi, the capital of the volatile Anbar province.
Some information for this report was provided by AFP.