US Defence Secretary Donald Rumsfeld has said that some captured Iraqi officials could have their punishments reduced in return for providing information.
Mahmud al-Tikriti is the most senior official to be captured
His comments came after British MPs suggested such a deal could be arranged in return for help in finding former Iraqi President Saddam Hussein, his sons, and weapons of mass destruction.
Earlier, the capture of Abid Hamid Mahmud al-Tikriti - Saddam Hussein's presidential secretary and one of his closest confidants - was announced.
He was number four on the US list of the 55 "most wanted" Iraqis, and the most senior figure to be detained to date.
Responding to a reporter's question about the possibility of "plea bargains" for former regime members, Mr Rumsfeld said: "We've thought about it a good deal and it is a perfectly reasonable proposal that those individuals are making, and the lawyers have been considering it.
"Those judgments are the kinds of things that the Justice Department and the Central Intelligence Agency will make with respect to the people."
The US administration built its case for war on Iraq around its contention that Iraq had weapons of mass destruction, and has recently faced increasing political and public questioning of its motives for war.
But a senior Pentagon official, Douglas Feith, told the BBC's World Today programme that weapons of mass destruction would be found.
Mr Feith said the fact that there was as yet no substantial evidence of a weapons programme did not mean that the war was wrong.
Meanwhile, President George W Bush has defended the British Prime Minister, Tony Blair, against accusations that he embellished intelligence reports to support his case for war.
Mahmud al-Tikriti is considered to have been third in power only to the president and his younger son, Qusay.
Former presidential secretary
Fourth on most wanted list
Believed to have knowledge of suspected weapons sites
The circumstances of his capture have not been disclosed.
BBC Washington correspondent Justin Webb says Mahmud al-Tikriti can expect to be put on trial for alleged crimes against humanity.
His detention brings to 32 the number of Iraqis on the US list of 55 "most wanted" members of the former regime who have been caught.
Mahmud al-Tikriti's arrest is reported to have coincided with a series of raids around Tikrit, Saddam Hussein's home town north of Baghdad.
US troops found $8.5m in cash, up to 400 million Iraqi dinars and an undetermined amount of British pounds and euros in swoops on two farmhouses, Major General Ray Odierno of the 4th Infantry Division told reporters.
Meanwhile, two protesters and a US soldier have been killed in continuing violence in Baghdad.
US troops opened fire on a crowd of former Iraqi soldiers protesting against the non-payment of salaries, killing two people.
The US military said its forces fired in self-defence after people in the crowd started throwing rocks.
The BBC's Pentagon correspondent, Nick Childs, says American military commanders are trying to play down the impact of the opposition they are facing.
General Odierno said the attacks on US forces have been unco-ordinated, ineffective and militarily insignificant.
In a separate incident, also in Baghdad, a US soldier was killed as he guarded a petrol station.