The Pentagon has described Abd al-Hadi al-Iraqi, who has been detained and transferred to Guantanamo Bay, as a trusted lieutenant of al-Qaeda leaders Osama Bin Laden and Ayman al-Zawahiri.
Abd al-Hadi al-Iraqi's role in Iran was a key interest of the US
The US said Mr Hadi, whose real name was Nashwan abd al-Razzaq abd al-Baqi, was arrested as he tried to return to Iraq.
However, no other details have been released on his capture.
A US intelligence official, speaking on condition of anonymity, told the BBC that Mr Hadi had a long-standing and deep awareness of al-Qaeda's training activities and operational planning.
Mr Hadi, a man with a reputation for being a skilled and intelligent commander, had been in CIA custody since late 2006, the official said.
US intelligence services were particularly interested in his activities in relation to Iran, he said.
It believes in addition to supporting terrorist activities in Iraq, Mr Hadi was involved in "causing problems in Iran as well", he said.
Mr Hadi wanted Iranian al-Qaeda operatives to be more aggressive in their attacks in Iraq and possibly in Iran as well, the official said.
Mr Hadi has cooperated during interrogation, providing insights into al-Qaeda's command and control structure, the official said.
Mr Hadi was born in Mosul, Iraq, in 1961, the Pentagon said.
Abu Musab al-Zarqawi was believed one of Mr Hadi's key links
He was formerly a major in the Iraqi army in the 1980s.
The US said he was in Afghanistan for 15 years, but that he had been a member of al-Qaeda "since the late 1990s".
Prior to the suicide plane attacks on the US in September 2001, Mr Hadi was a member of the 10-strong Shura Council - the advisory body to Bin Laden.
Mr Hadi was also said to be a member of al-Qaeda's Military Committee, which oversaw the group's operations and training.
The Pentagon said he was a direct link between al-Qaeda leaders and the Taleban in organising attacks on US forces in Afghanistan.
Later he was also believed to be a key link between Bin Laden and the head of the group in Iraq, Abu Musab al-Zarqawi, who has since been killed.
Mr Hadi was also alleged to have been involved in plots to assassinate Pakistani President Pervez Musharraf and an unnamed UN official.