Foley announced through his lawyer that he had been battling alcoholism and had checked into an unidentified rehabilitation facility for treatment over the weekend.
"I strongly believe that I am an alcoholic and have accepted the need for immediate treatment for alcoholism and other behavioral problems," Foley said. "I deeply regret and accept full responsibility for the harm I have caused."
Florida Republicans picked state Rep. Joe Negron to replace Foley as the party's candidate in the West Palm Beach district, which is largely Republican.
The FBI has opened a preliminary inquiry to determine whether Foley's sexually suggestive e-mails violated federal law. Foley abruptly resigned from Congress on Friday after reports surfaced that he'd sent sexually charged electronic messages to boys working as pages.
Several pages have said Foley paid them a great deal of attention, more than his colleagues, in fact.
"Congressman Foley, I would say, was the winner in knowing the most pages and knowing them on a first-name basis," Blake Yocom, a former page, told CBS News correspondent Sharyl Attkisson.
Yocom was a page in 2003. He says at the time he wrote off Foley's interest in him and his young colleagues as overly friendly ... but it's much creepier in retrospect.
"A few of my close friends, pages, suspected that Congressman Foley was a homosexual. But we never expected pedophilia out of Congressman Foley," he told CBS News.
Foley was so interested in the page program that he once praised them in a tearful speech on the House floor as their time in Washington ended. Speaking in 2002 about a contest where the pages bid for the privilege of lunch with him, Foley said he and a page who won the bid headed for Mortons for lunch, together. "And so we proceeded to cruise down in my BMW to Morton's. And all of this story is meant to make you all feel jealous that you were not the high bidders," Foley said.
The FBI may not have seen it, but some 16-year olds sensed Foley was crossing a line that other Congressmen didn't.
"I have friends, Matt and Tim, and he would always see them first and say 'Hi, Matty. Hi, Timmy.' And we'd say 'That's pretty funny of him to say that, you know, that's kind of weird," Jenna Kelsey, a former House Page, told CBS News.
The Washington Post also reported that Patrick McDonald, a former House page, said that he had learned that three or four pages in his 2001-2002 class had received inappropriate e-mails from Foley.
McDonald, 21, said he had seen Foley e-mails sent to another page during a 2003 page reunion and recalled saying, "If this gets out, it will destroy him."
CBS News correspondent Bob Fuss reports that there are indications that members of the House leadership — all the way up to Speaker Dennis Hastert — were told about this more than a year ago. While they will admit they knew about the inappropriate e-mails, they say they never knew anything about sexually explicit ones.
"If it's proven that leaders in Congress did nothing, nothing to protect those children — those 15- and 16-year-olds — those members of Congress should resign their leadership positions," Rep. Sherrod Brown, D-Ohio, said, CBS News' Aleen Sirgany reports.
House Republicans, meanwhile, scrambled to limit political damage from the unfolding scandal following news that House leaders knew for months about Foley's inappropriate overtures toward the pages.
Hastert said Monday that no Republican leaders saw lurid Internet exchanges from Foley to pages and that he would have demanded the Florida Republican's expulsion if he had known about them.
"As a parent and speaker of the House, I am disgusted," Hastert, R-Ill., told reporters after holding a meeting at the Capitol in the wake of the disclosure of the e-mails in 2003 to a page, which led to Foley's resignation last Friday.
Hastert, in a letter to Attorney General Alberto Gonzales, asked the Justice Department to "conduct an investigation of Mr. Foley's conduct with current and former House pages."
Democrats are demanding that investigators determine whether Republican leaders tried to cover up Foley's actions for political reasons.
"The attorney general should open a full-scale investigation immediately," Senate Democratic leader Harry Reid of Nevada said in a statement, including whether GOP leaders "knew there was a problem and ignored it to preserve a congressional seat this election year."
In related developments:
was "outraged and disgusted with Congressman Mark Foley's actions."
FBI cyber-sleuths are looking into the text of some of Foley's messages, checking to see how many e-mails and instant electronic messages were sent and how many computers were used, according to a law enforcement official, who spoke on condition of anonymity because of the ongoing investigation.
Ironically, Foley, who is 52 and single, could be found to have violated a law that he helped to write as co-chairman of the Congressional Missing and Exploited Children's Caucus.
"Republican leaders have admitted to knowing about Mr. Foley's outrageous behavior for six months to a year, and they chose to cover it up rather than to protect these children," said House Democratic leader Nancy Pelosi of California.
Rep. Thomas Reynolds of New York, head of the House Republican election effort, said he told Hastert months ago about the allegations involving a 16-year-old boy from Louisiana.
Hastert acknowledged that his staff had been made aware of concerns about what they termed "over-friendly" e-mails Foley had sent to the teenager — including one requesting his picture — in the fall of 2005, and that they referred the matter to the House clerk.
But Hastert said those e-mails were not viewed as "sexual in nature" and that he was not aware of "a different set of communications which were sexually explicit ... which Mr. Foley reportedly sent another former page or pages."
Hastert asked the Justice Department to investigate "anyone who had specific knowledge of the content of any sexually explicit communications between Mr. Foley and any former or current House pages and what actions such individuals took, if any, to provide them to law enforcement."
A Pelosi spokeswoman, Jennifer Crider, said that Hastert "seems more concerned by who revealed the Republican leadership cover-up of Mr. Foley's Internet stalking of an underage child than he was about ensuring the children entrusted to the House were protected."
Congressional pages, a staple of Washington politics since the 1820s, are high school students who serve as gofers in the House and Senate.