Senior US politicians have called for a full inquiry into the conduct of former Congressman Mark Foley, who is accused of improper contact with staff.
Mr Foley was on the House caucus on missing and exploited children.
When confronted last week with sexually suggestive messages he had sent to boys, the Florida Republican resigned.
The FBI is examining the messages to see if any law has been broken.
The scandal has led to accusations that senior Republicans may have been aware of the communications but decided to ignore them.
Mr Foley - a member of the House of Representatives caucus on missing and exploited children - apologised when announcing his resignation on Friday.
House Speaker Dennis Hastert on Sunday urged the justice department to investigate Mr Foley's messages to pages - students serving as messengers and assistants to members of Congress.
Some messages were sexually explicit and at least one of the recipients was only 16 years old.
The White House and Democratic leaders in Congress also called for a criminal inquiry.
"There is going to be, I'm sure, a criminal investigation into the particulars of this case," said White House counsel Dan Bartlett - who called the allegations "shocking".
The BBC's Justin Webb in Washington says that over the weekend it has become clear that a minor embarrassment for the Republican party might become something far worse.
It has emerged that Mr Foley's behaviour had been known about for some time by senior colleagues, including the party's leader in the House.
They are now arguing about how much they knew - but the Democrats and some Republicans have called for the party's Congressional leaders to step down.
Our correspondent says the scandal could cost the Republicans votes in next month's mid-term elections - votes they cannot afford to lose.