"We will keep an open mind and we will continue to report credible evidence and responsible points of view as we try to answer the questions raised about the authenticity of the documents," CBS News Anchor Dan Rather said on 60 Minutes on Wednesday.
But Rather said CBS' critics have never attacked the thrust of the network's story: that Mr. Bush received preferential treatment to get into the National Guard and stay stateside during the Vietnam War, and failed to satisfy the requirements of his service.
"If we uncover any information to the contrary, rest assured we shall report that also," the embattled anchor said.
His report came after a , when top Republicans tried to tie the Kerry campaign to the disputed documents, called for a congressional investigation and for CBS to retract its story.
In question are memos purportedly written by Mr. Bush's late squadron commander, Lt. Col. Jerry B. Killian, indicating that he had been pressured to sugar coat Mr. Bush's performance and that the future president had ignored an order to take a physical.
Critics have said the documents look like they were written on a modern computer, not a 1970s typewriter. Skeptics have also said the memos contain stylistic differences with other documents attributed to Killian, dated information and improper military lingo. Meanwhile, associates of the Col. Killian are split on whether the content of the memos reflected his thinking at the time.
CBS on Wednesday flew Killian's former secretary, , 86, from Texas to New York for an interview. In the interview, Knox said she believed the documents were fake but their content accurately reflected Killian's opinions.
"I know that I didn't type them," she said. "However, the information in those is correct."
Rather's boss, CBS News President Andrew Heyward, also said CBS would redouble its efforts to determine the authenticity of the documents.
"Enough questions have been raised that we are going redouble our efforts to answer those questions," he said.
Several experts have said the documents, disclosed last week, are fakes, prepared on a modern computer rather than a typewriter in 1972 and 1973.
CBS on Wednesday released letters from two document examiners who reviewed the memos.
Forty members of the House signed their own letter, accusing the network of deception. The letter asked CBS if the documents are authentic, and why the network won't say how it got them, reports CBS News Correspondent Wyatt Andrews.
"I think at the very least CBS should characterize the source," said Rep. Roy Blunt, R-Missouri. "I think it's amazing that they haven't already done that."
CBS News officials said the memos came from a confidential source, and in a statement cited longstanding journalistic practice in refraining from describing how they obtained the documents.
Bill Burkett, a retired National Guard officer from Texas, has been cited in reports in Newsweek and The New York Times as a possible source for CBS' report. Burkett is not talking to the press.
Burkett told The Associated Press in February that he witnessed efforts in 1997 in Texas by allies of then-Gov. Bush to destroy military records that might "embarrass the governor."
Robert Strong, another former National Guard officer, told The Washington Post on Thursday that at least one of the documents used by CBS bore a faxed header indicating it had been sent from a Kinko's copy shop in Abilene, Texas, about 20 miles from Burkett's home.
Strong said he was shown copies of the documents three days before the 60 Minutes broadcast on which he appeared.
CBS News says the original report used several different techniques to make sure the memos were genuine, including talking to handwriting and document analysts and other experts who strongly insist that the documents could have been created on a typewriter in the 1970s. After the controversy arose, CBS has interviewed other experts who support the network's view.
Wednesday's CBS statement came as two document examiners used by CBS told ABC News they questioned aspects of the story before it aired.
Emily Will said she told the network before the report aired that she questioned handwriting in the documents she was shown and whether it could have been produced by a typewriter. Linda James told ABC, "I did not authenticate anything and I don't want it understood that I did."
CBS News said that Will and James played only a "peripheral role" in assessing the documents and ultimately deferred to another expert who has seen all four documents, Marcel Matley. In a letter produced by CBS News, Matley says he believed the same person signed "Jerry B. Killian" on all the documents he saw.
Another expert, James Pierce, wrote, "the balance of the Jerry B. Killian signatures appearing on the photocopied questioned documents are consistent and in basic agreement."
Republican National Committee spokesman Jim Dyke linked the Kerry campaign to the memos when he said a new Democratic National Committee video was "as creative and accurate as the memos they gave CBS."
Responded Phil Singer, Kerry campaign spokesman: "It's ridiculous. We didn't give CBS anything."
White House press secretary Scott McClellan said: "The one thing that is not under question is the timing of these orchestrated attacks by the Democrats on the president's service. And these are old, recycled attacks."
Indeed, questions have been raised for years about Mr. Bush's entry into and service in the Guard, especially a period from mid-1972 to mid-1973 for which there is conflicting evidence Mr. Bush performed his duties. It is known that he missed a flight physical during that time and asked for permission to transfer to an Alabama unit to work on a campaign there.
Featured in the original 60 Minutes segment was former Texas Lt. Gov. Ben Barnes, a Democrat who claims he pulled strings to get Mr. Bush into the Guard in 1968. And released by the Pentagon this summer show no indication Mr. Bush drilled with the Alabama unit during July, August and September of 1972, The Associated Press has reported.
The White House contends Mr. Bush received no favorable treatment and fulfilled his duties.