The ethics committee of the US House of Representatives has opened an inquiry into a scandal over a former Republican congressman's explicit e-mails.
Mark Foley denies ever having sexual contact with a minor
The scandal was triggered by recent revelations that Mark Foley had sent lurid e-mails to young male staff.
But the committee will also investigate claims that Republican leaders knew about Mr Foley's activity years ago but chose not to act.
House Speaker Dennis Hastert apologised over the row, but ruled out resigning.
Mr Foley, a Republican member of the House caucus on missing and exploited children, resigned last week after the revelations that he had sent sexual messages to House pages.
The scandal is dominating US politics ahead of the mid-term elections next month.
If claims that the Republican leadership knew about Mr Foley's messages were to be proven, the party's chances of holding on to Congress after the poll would be greatly reduced, the BBC's Justin Webb in Washington says.
The ethics committee, which has five Democrats and five Republicans, held its first meeting on Thursday.
It said it hoped to finish the inquiry within weeks.
The committee will examine who became aware of the allegations and when, although it has no jurisdiction over Mr Foley.
It has already ordered almost 50 subpoenas for evidence.
Committee chairman Doc Hastings said: "The American people, and especially the parents of all current and former pages, are entitled to know how this situation was handled."
The youngest recipient of the suggestive e-mails is said to have been 16-year-old.
Mr Foley - through his lawyer - has said he was gay but denied any sexual contact with minors.
US media earlier this week revealed a possible romantic link with a young congressional worker and said warnings about Mr Foley's conduct went as far back as 1995.
'Buck stops here'
On Thursday, Mr Hastert said he was "deeply sorry" over the scandal.
House leader Hastert has received backing from the president
"We are taking responsibility because the buck stops here," he said.
But he also said: "I haven't done anything wrong."
Mr Hastert insisted he would stay as speaker, but asked the ethics committee to consider new rules on inappropriate contact with pages - high school students appointed to help with administrative work.
Mr Hastert said he had asked for investigations by the ethics committee, the FBI, the justice department and the state of Florida.
He also denied any early knowledge of the e-mails, saying: "I learned of this last Friday... I don't know who knew what or when - that's why we've asked for an investigation."
"If members of my staff didn't do their job, we will act appropriately," he said.
He also suggested that the Democrats were using this scandal as "another political tactic".
His comments came after Kirk Fordham, Mr Foley's chief-of-staff until 2004, said he had warned the speaker's office of Mr Foley's behaviour more than three years ago.