Meanwhile, a former secretary in the guard said Tuesday she believed the documents in question were fake, although they accurately reflected the thoughts of one of Mr. Bush's commanders.
Republican Congressman Chris Cox of California called for a congressional probe of the network's use of the documents, saying there's a "growing abundance of evidence that CBS News has aided and abetted fraud." CBS hasn't commented on the request.
Last week CBS News 60 Minutes reported that documents from one of Mr. Bush's commanders, Lt. Col. Jerry B. Killian, indicated Mr. Bush didn't follow orders to take a physical and that Killian was being pressured to sugarcoat his performance ratings. Mr. Bush's father was a Texas congressman at the time. The network has not revealed how it obtained the documents.
Questions were immediately raised about the documents' legitimacy, with some believing they were produced by a computer not available at the time.
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 – as opposed to a modern-day word-processing software program.
CBS has also said its story about Mr. Bush's guard service relied on much more than documents. Featured in the segment was former Texas Lt. Gov. Ben Barnes, a Democrat who claims he pulled strings to get Mr. Bush into the Guard in 1968.
CBS News on Tuesday said the report by Dan Rather did not rely on assessments made by the two examiners quoted in the ABC report, and found it notable the secretary affirmed the content of the documents.
"We continue to believe in this story," said CBS News senior vice president Betsy West.
Emily Will, a documents examiner from North Carolina hired by CBS, 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.
Her main concern was that she was not provided a known sample of the signature to use for comparison.
Will said she e-mailed a CBS producer and urged her the night before the broadcast not to play up that a professional document examiner had authenticated the papers.
"I did not feel that they wanted to investigate it very deeply," Will told ABC News.
Another expert hired by CBS, Linda James of Plano, Texas, told ABC that "I did not authenticate anything and I don't want it understood that I did."
James told AP late Tuesday she raised similar concerns about signature samples.
"I really pressed that because I knew that other document examiners looking at the same documents would have a real problem authenticating these," she said.
West said Will did not contact the network the night before the report aired.
"I am not aware of any substantive objections raised," she said. "She did not urge us to hold the story."
James told CBS News that she needed to know more about the documents before rendering any judgments, West said. CBS contacted five document experts before the report aired and two since, and continues to report the story, the network said.
CBS News said that Will and James played only a "peripheral role" in assessing the documents, and had seen only one of the four used in the report. Ultimately they deferred to another expert who has seen all four documents, Marcel Matley, and who continues to back up CBS' account.
However, Matley has told CNN, The Washington Post and other media organizations that his work was limited to verifying that the signatures on the memos came from the same source. He did not, he says, claim that the documents themselves were authentic.
Killian's former secretary, 86-year-old Marian Carr Knox, also questioned the documents in an interview with The Dallas Morning News.
"These are not real," Knox said in a story posted Tuesday on the newspaper's Web site. "They're not what I typed, and I would have typed them for him."
Knox told the newspaper she did not recall typing the memos, but that they echoed Killian's views on Mr. Bush. She said he retained memos for a personal "cover his back" file he kept in a locked drawer of his desk, but she was not sure what happened to them when he died in 1984.
CBS News spokeswoman Sandra Genelius said CBS did not believe Knox was a documents expert and that the network believes the documents are genuine.
"It is notable that she confirms the content of the documents, which was the primary focus of our story in the first place," Genelius said.
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.
On Tuesday, a private anti-Bush group called Texans for Truth offered a $50,000 reward for "anyone who can prove Bush's claim that he fulfilled his service requirements."
The White House contends Mr. Bush received no favorable treatment and fulfilled his duties. The president on Tuesdayconference in Las Vegas, Nev. He honored the sacrifice of National Guardsmen in Iraq and Afghanistan, and expressed pride in his stateside stint in the Air National Guard during the Vietnam War. He did not address the controversy over his service.
show no indication Mr. Bush drilled with the Alabama unit during July, August and September of 1972, The Associated Press has reported.
The Boston Globe newspaper has reported that on two occasions while serving in the Guard, Mr. Bush signed documents in which he