Saddam Hussein's half-brother Barzan Ibrahim al-Tikriti has been captured by US-led special forces in Baghdad, says US Central Command.
Barzan al-Tikriti as pictured by the US military
US Army Brigadier-General Vincent Brooks said the former head of Iraqi intelligence, was seized alone following a tip-off by Iraqis.
He said the development sent out a powerful message.
"The capture demonstrates the coalition's commitment to relentlessly pursuing the scattered members of a fractured regime," said General Brooks.
"Barzan is...an adviser to the former regime leader with extensive knowledge of the regime's inner working," he added.
The BBC's Dominic Hughes at US Central Command in Qatar says the arrest is particularly significant, as the Americans rely on senior Iraqi officials to provide details of Iraq's weapons programmes.
CAPTURED IRAQI LEADERS
51. Watban Ibrahim al-Tikriti
52. Barzan Ibrahim al-Tikriti
55. General Amir al-Saadi
It had been thought that Barzan al-Tikriti was killed last week when his home was targeted by an air strike.
"We are currently asking a number of questions and finding out whatever we can from this capture," said General Brooks.
"He will be questioned over coming days and information may point us in the direction of the inner workings of the regime."
Barzan al-Tikriti ran the Iraqi intelligence service between 1979 and 1983 before becoming Iraq's ambassador to the United Nations.
He is number 52 on the American list of 55 most wanted Iraqis and the second of Saddam Hussein's three brothers to be taken.
Watban Ibrahim al-Tikriti was arrested near the Syrian border at the weekend.
American agents questioning Barzan al-Tikriti will be trying to tap into his extensive knowledge of the inner workings of Saddam Hussein's regime, our correspondent says.
Watban Ibrahim al-Tikriti was arrested at the weekend
Human rights groups regard him as one of the most enthusiastically sadistic members of Saddam's inner circle.
British-based organisation Indict alleges that, as head of intelligence, he was responsible for a catalogue of mass murder and torture who personally participated in a number of atrocities.
They accuse him of participating in the detention and murder of several thousand men who disappeared from the Barzani tribe in 1983.
In the same year, he allegedly carried out the destruction of two villages and orchestrated the subsequent murder or deportation of the inhabitants.
"If this news is true, we are very happy," said Chief Executive Officer of Indict, Charles Forrest told BBC News Online.
Barzan al-Tikriti was reported to have tried to get political asylum in the United Arab Emirates in 1999 after a feud with other members of Saddam Hussein's family.