US House of Representatives Speaker Dennis Hastert has apologised over a sex scandal involving a fellow Republican, but said he will not quit.
House leader Hastert has received backing from the president
The embattled speaker denies reports that he had prior knowledge of lurid e-mails sent by ex-Congressman Mark Foley to boys, but took no action.
Mr Hastert said he took responsibility but accused opposition Democrats of using the scandal for political ends.
The statement came as the House ethics panel began an inquiry into the case.
Mr Foley, a Republican member of the House caucus on missing and exploited children, resigned last week after revelations that he had sent sexual messages to young men on his staff.
The scandal is dominating politics ahead of the mid-term elections and recent opinion polls suggest it may be having an effect.
'High calibre' person
Mr Hastert called a news conference on Thursday after Kirk Fordham, Mr Foley's chief of staff until early 2004, said he had warned the Speaker's office of Mr Foley's behaviour more than three years ago.
The comments added to the pressure, including from within Republican circles, on Mr Hastert.
Mr Hastert insisted he would stay as speaker, but asked the Ethics Committee to consider new rules on inappropriate contact with page workers - high school students appointed to help with administrative work in the House.
Mr Hastert said: "I am deeply sorry that this has happened. We are taking responsibility because the buck stops here."
But he also said: "I haven't done anything wrong."
Mr Hastert said he had asked for investigations by the House Ethics Committee, the FBI, the Justice Department and the state of Florida.
He said he wanted a "high calibre" person to investigate the page system.
"We want a system in place to ensure this never happens again and we will do everything possible to make sure the programme is safe," Mr Hastert said.
He 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."
He added: "If members of my staff didn't do their job, we will act appropriately."
He also suggested that the Democrats were using this scandal as "another political tactic".
The House Ethics Committee, which has five Democrats and five Republicans, held a meeting on Thursday and announced it had begun an inquiry.
Mark Foley denies ever having sexual contact with a minor
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 years old.
Mr Foley denied ever having sexual contact with a minor, his lawyer said - adding that his client had declared he was gay and had been abused in his early teens.
On Wednesday, US media revealed a possible romantic link with a young congressional worker and that warnings about Mr Foley's conduct went as far back as 1995.
The latest opinion poll suggests the Democrats are within reach of retaking the House of Representatives, leading in 11 of 15 vital Republican-held seats on 7 November.