Mark Foley, 52, quit his seat in disgrace last week after revelations that he exchanged intimate electronic messages with young boys, including some that were sexually explicit.
One exchange involved a former page in the US House of Representatives, whom Foley asked to send a picture. The boy was 16 at the time of his contact with Foley, a representative from Florida.
With leaders of President George W Bush's centre-right party scrambling to limit the election-year damage, the FBI opened a preliminary investigation into possible criminal wrongdoing.
'The FBI is conducting as assessment to determine whether there's been any violation of federal law,' agency spokesman Stephen Kodak said Monday. He declined to elaborate.
The growing scandal is ill-timed for Bush, implicating a member of his party less than six weeks before mid-term elections where the Republicans face a tight battle to keep their majority in both houses of Congress.
Bush's chief spokesman urged the Republican-led House to clear up how Foley could get away with his behaviour.
'Members of the leadership as well as former representative (Foley)... look, it's a terrible story and I think people deserve to figure out what went on,' spokesman Tony Snow told ABC television Monday.
