The growing scandal is ill-timed for Bush, implicating a member of his party five weeks before Congressional elections where the centre- right Republicans are battling to keep their majority in both houses.
Mark Foley, 52, a representative from Florida, quit his seat in disgrace last week after revelations that he exchanged intimate e- mails and instant messages - including some that were sexually explicit - in 2003 with boys who had worked as pages in Congress.
While the FBI examined whether federal laws may have been broken, a letter surfaced Monday in which Foley expressed regret and said he had checked into alcoholism rehab.
'Painfully, the events that led to my resignation have crystallized recognition of my longstanding significant alcohol and emotional difficulties,' Cable News Network (CNN) quoted the letter as saying.
Pages come to Washington from across the US to help in the House and Senate with clerical and administrative duties. One boy, whom Foley allegedly asked in an e-mail to send a picture, was 16.
Reports that other members of the House of Representatives knew of Foley's behaviour but were slow to intervene could be particularly damaging for Republicans, since they control the chamber.
House Speaker Dennis Hastert, in an extraordinary move, has asked US justice officials to examine whether lawmakers may have mishandled the allegations against Foley.
That followed calls by opposition Democrats for a quick inquiry. The FBI, meanwhile, said it 'is conducting an assessment to determine whether there has been any violation of federal law.'
Hastert said he was 'outraged and disgusted' by Foley, a 11-year House veteran who championed tough laws against sex offenders and child pornography.
But a former Republican party strategist said the fallout for Bush's party would be worse than from renewed debate about the administration's handling of the Iraq war, sparked by a damning book by US star journalist Bob Woodward.
Voters 'do understand when someone is sending perverted e-mails to a young person,' Ed Rollins told CNN. 'This is going to turn off Republican voters.'
Bush's chief spokesman, Tony Snow, urged House Republican leaders to clear up the Foley case.
'Look, it's a terrible story and I think people deserve to figure out what went on,' he told ABC television Monday.
Congressional pages usually do not run into trouble, though in 1983 Congress censured two lower-house members for sexual misconduct - one with a boy, the other with a girl, the New York Times reported.
