A half-brother and one-time aide to former Iraqi leader Saddam Hussein has been captured, the Iraqi interim government has said.
Mr al-Tikriti was said to be helping direct the insurgency from Syria
It accused Sabawi Ibrahim al-Hasan al-Tikriti of planning and carrying out terrorist acts.
He is high on a list of the 29 most wanted insurgents, and the US military was offering a $1m reward for him.
Only 11 of the 55 most wanted former regime members have escaped capture or death at the hands of US-led forces.
Mr Tikriti, a maternal half-brother of Saddam Hussein, was number 36 on the list of former Baathist officials issued by the Americans.
He is the first to be captured since February 2004.
No details have been given about where or when the capture took place.
A statement from interim Prime Minister Iyad Allawi's office said he had "killed and tortured Iraqi people".
It accused him of "planning, supervising, and carrying out many terrorist acts in Iraq".
SABAWI IBRAHIM AL-HASAN AL-TIKRITI
Accused of directing insurgency from Syria
Presidential adviser during Saddam Hussein's regime
Six of diamonds in US most wanted playing cards
Brothers Watban and Barzan also arrested
The arrest "shows the determination of the Iraqi government to chase and detain all criminals who carried out massacres and whose hands are stained with the blood of the Iraqi people, then bring them to justice to face the right punishment".
The detainee was head of the regime's intelligence services during the 1991 Gulf War, and then chief of general security until 1996 before becoming a presidential adviser to Saddam Hussein.
Just last month Iraqi government officials named him as one of a group of former regime figures they said were directing and financing insurgency operations in Iraq from bases inside neighbouring Syria.
They had asked the Syrians to extradite him and others.
The BBC's Jim Muir in Baghdad says the former aide's detention will encourage Iraqi and American officials to believe they are making progress in their efforts to tackle the insurgency.
Mr Tikriti's brothers Watban and Barzan are already being held at a US facility outside Baghdad.
They are expected to be tried in the next few months at a special tribunal along with other members of the regime.
Prosecutors are currently working to compile specific evidence against them.
News of the capture came as the US Army's Third Infantry Division, which captured Baghdad from Saddam Hussein's forces two years ago, formally took over responsibility for the capital as part of a troop rotation.
The division was heavily criticised at the time for failing to stop the looting that plagued Baghdad just after the war.