US Attorney General Alberto Gonzales, the country's top law enforcement officer, has admitted mistakes were made in the dismissal of key personnel.
Mr Gonzales denied knowing specific details of the firings
Mr Gonzales "accepted responsibility" for the way eight US attorneys were fired, but denied personal involvement.
Congress has been investigating whether the firing of the prosecutors - nearly one in 10 nationwide - was politically motivated. Some have called it a purge.
Mr Gonzales's top aide resigned on Monday as the scandal gathered steam.
Charles Schumer, the number-three Democrat in the Senate, said Mr Gonzales himself should resign.
He condemned Mr Gonzales's statement that he had not been personally involved in the firings: "To say that decisions were delegated... that is a sorry excuse."
He said the firings had "shaken the integrity of the US attorneys' offices to the core".
White House involvement
Mr Gonzales said his chief of staff Kyle Sampson had been in charge of the process. Mr Sampson handed in his resignation on Monday night.
Mr Gonzales said on Tuesday that two years ago the White House had proposed dismissing all 93 US attorneys.
Serve at the discretion of the president, with the advice and consent of the Senate
Prosecute criminal cases brought by the government
Prosecute or defend civil cases in which the government is a party
Collect debts owed to the government
Source: US Department of Justice
That appeared to confirm a report in the Washington Post newspaper that President George W Bush's former counsel, Harriet Miers, suggested the idea to the justice department.
Mr Gonzales said he rejected the idea as impractical: "I felt that was a bad idea and that was disruptive."
But, he said, the suggestion led to an evaluation of which attorneys were "strong performers" and which were not, and ultimately to the dismissal of eight.
Critics - including some of the former prosecutors themselves - say they were fired for political reasons or to make way for cronies of the administration to take their place.
Judiciary committees in both chambers of Congress are expected to deepen their inquiries into the dismissals of the prosecutors.
Mr Gonzales said he was aware in general terms of what was happening but never saw documents or participated in discussions about the firings.