"Ultimately ... the buck stops here," the Republican speaker said, using the phrase of a Democratic president, Harry Truman.
Hastert held to his assertion that he did not know about ex-Rep. Mark Foley's e-mails and sexually explicit computer messages to former pages until the scandal broke last week.
The ethics committee promised to finish its investigation in weeks, not months, but it was unclear whether that would occur before the Nov. 7 election. Hastert's handling of the issue has brought harsh criticism from some fellow Republicans and conservative activists.
An AP-Ipsos poll found that about half of likely voters say recent disclosures of corruption and scandal in Congress will be very or extremely important. That group is much more likely to vote Democratic.
The speaker, at a news conference, mixed a newfound contriteness with defiance.
"Could we have done it better? Could the page board have handled it better? In retrospect, probably yes. But at the time what we knew and what we acted upon was what we had."
But he also vowed to win re-election and run for House speaker again.
Privately, Republicans say that Hastert's mea culpa was the very least he could do, but they're far from sure it will be enough, CBS News correspondent Gloria Borger reports. On the record, they're trying to be upbeat.
"Hopefully he's cleared the air so members and challengers can have this off the front pages of the paper," Rep. Ray Lahood, R-Ill., told Borger.
Meanwhile, President Bush called Hastert to thank him for how he has handled the situation. The White House says that Mr. Bush praised Hastert for "making a clear public statement that said the House leadership takes responsibility and is accountable."
While the Committee on Standards of Official Conduct — the ethics committee — is investigating potential violations of House rules, the Justice Department appeared to be moving with dispatch in its criminal investigation.
Timothy Heaphy, a lawyer for ex-Foley chief of staff Kirk Fordham, said his client had just met with the FBI. Fordham emerged as a key figure Wednesday when he told reporters that he had talked three years ago with top aides to Hastert about Foley's conduct with pages. His comments pushed back the time when information may have reached the speaker's office.
CBS News correspondent Sharyl Attkisson reports that the FBI also visited former page Tyson Vivyan, who said Foley sent him sexually charged instant messages, but only after he left the page program. He told CBS News the messages were "referencing genitalia, referencing specific acts between the two of us, things like that."
Since Vivyan wasn't a minor, the FBI told him it didn't appear Foley had violated any laws in his case.
Ethics committee chairman Doc Hastings, R-Wash., and ranking Democrat Howard Berman of California would provide no details on the subpoenas but told a news conference the committee was seeking both testimony and documents.
Hastert spokesman Ron Bonjean said the speaker had not yet received a subpoena from the ethics committee but was willing to testify. "If the ethics committee asks him to, of course," Bonjean told The Associated Press.