US lobbyist Jack Abramoff has pleaded guilty to fraud, paving the way for him to co-operate in an inquiry into corruption in Congress.
Jack Abramoff faces separate charges in Miami
Mr Abramoff has also pleaded guilty to conspiracy and tax evasion.
He and an associate are said to have defrauded several indigenous tribes of millions of dollars.
The money was allegedly spent on some 20 lawmakers and aides, who are now the focus of a US justice department investigation.
Mr Abramoff's former business partner, Michael Scanlon, pleaded guilty in November.
The lobbyist is also facing separate charges in Miami over the purchase of a fleet of gambling boats, a case in which he is also expected to plead guilty.
The indictment against Mr Abramoff and Mr Scanlon accuses them of putting millions of dollars, wrongfully claimed from indigenous tribes, into a fund for bribing lawmakers.
Gifts and favours lavished upon the lawmakers included golfing trips, dinners and sports tickets, according to the indictment.
Mr Abramoff told the judge at Tuesday's hearing: "Words will not ever be able to express my sorrow and my profound regret for all my actions and mistakes."
He agreed with the judge when she said he had been part of a conspiracy involving "corruption of public officials" and acknowledged that he had helped deliver favours and gifts "in exchange of certain official acts".
Prosecutors are trying to implicate top Republicans in the scheme, including Tom DeLay, who stepped down as the House of Representatives majority leader to fight the allegations earlier this year.
Mr DeLay denies charges of money-laundering.