The US justice department has said it intends to pursue senior politicians suspected of taking bribes from prominent lobbyist Jack Abramoff.
Jack Abramoff faces separate charges in Miami
Abramoff pleaded guilty to charges of fraud, conspiracy and tax evasion.
Under the terms of a plea bargain, Abramoff will co-operate with a probe into alleged corruption among lawmakers and their staff.
Abramoff had ties to prominent members of the Republican Party. Some Democrats are also said to be implicated.
The BBC's Justin Webb in Washington says the case is an unexploded bomb under both houses of Congress.
The lobbyist is accused of using gifts of money, foreign trips and lavish meals to buy political influence.
His former business partner, Michael Scanlon, pleaded guilty in November.
The indictment against Abramoff and Scanlon accuses them of putting millions of dollars, wrongfully claimed from indigenous tribes, into a fund for bribing lawmakers.
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.
Justice department officials are believed to be focusing on as many as 20 members of Congress and aides.
"This investigation continues... however long it takes, wherever it leads," assistant attorney general Alice Fisher said.
Our correspondent says Abramoff has apparently promised to reveal not just who was paid and how much, but also what the understandings were that led to the transfer of favours and money.
If he claims credibly that he was expecting political action to be taken in return for cash, then a number of senior politicians may have committed serious crimes and could end up in jail.