Updated

CBS News, dogged over questions about the authenticity of memos apparently showing that President Bush shirked his National Guard service more than 30 years ago, said on Wednesday that it stood by the basic content of its report.

But the network did seem to acknowledge that the memos may have been forgeries, and said it would "make every effort to resolve the contradictions and answer the unanswered questions about the documents and will continue to report on all aspects of the story."

Numerous experts have said the memos in question were likely not written on a typewriter in the early 1970s, as CBS claims, but on a modern personal computer using word-processing software.

Marion Carr Knox (search), the former secretary of Bush's Texas Air National Guard (search) commander, said she typed all Lt. Col. Jerry Killian's (search) correspondence from 1956 to 1979 and that while the memos CBS showed were forgeries, the sentiments reflected in them about Bush were accurate.

"I know that I didn't type 'em," Knox, 86, told CBS News' "60 Minutes" in an interview set to air Wednesday night. "However, the information in those is correct."

She said Killian 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.

"It is notable that she confirms the content of the documents, which was the primary focus of our story in the first place," said CBS News spokeswoman Sandra Genelius.

The White House has said Bush stands by his service, is proud of it and is not concerned with the question of the documents' authenticity.

Lawmakers Take Sides

A House memo being circulated by GOP Whip Roy Blunt of Missouri calls on CBS News President Andrew Heyward to retract the network's story and to disclose the identities of the people used to help "authenticate" the documents.

"We are writing to express our dismay that CBS has become part of a campaign to deceive the public and to defame the president," says the letter, which FOX News has learned has been signed by about 40 House members.

"To date, CBS's response to the specific and devastating criticisms of the accuracy of its reporting has been to question the motives of its critics, to offer half-truths in its own defense, to refuse to disclose crucial evidence, and to circle the wagons … CBS reporters would not accept such behavior from public officials like ourselves, and we cannot accept it from them."

House Democratic Leader Nancy Pelosi said the call is the Republicans' "latest attempt to intimidate the news media [and] is a waste of taxpayer money and an egregious example of how this Republican House only exercises its oversight responsibility for partisan political reasons."

Meanwhile, Rep. Chris Cox (search), R-Calif., chairman of the House Policy Committee and of the Homeland Security Committee, called on Energy and Commerce Subcommittee on Telecommunications Chairman Fred Upton, R-Mich., to investigate CBS News' use of "apparently forged documents concerning the service record of President George W. Bush intended to unfairly damage his reputation and influence the outcome of the 2004 presidential election."

"Despite the growing abundance of evidence that CBS News has aided and abetted fraud, the network has declined to reveal the source of the disputed documents," Cox wrote in the hand-delivered letter. "Given the shortness of time between now and the election, which the apparent fraud is meant to influence, and the even shorter time before Congress is scheduled to adjourn, I strongly urge that the subcommittee move with all deliberate speed to uncover the facts."

Sen. Richard Shelby, R-Ala., also told FOX News that the documents look "more than suspect" and "have no place in the political contest."

"I think Congress needs to look at things … what the news media does at times is very important and if they're fraudulent documents, and somebody has reason to believe that they were or doubted their authenticity, they ought to be very, very careful before they use them," Shelby said. "CBS obviously has a problem with their source."

Lawmakers took turns chastising CBS on the Senate floor Wednesday and urging everyone to move on from the issue.

"We're not electing CBS president of the United States so it would be my hope that whatever the circumstances ... if they've made a mistake ... they admit it," said Sen. Lamar Alexander, R-Tenn. "We politicians have learned the hard way that's the thing to do — we're usually taught that by the media and then let's get on with it, let's move."

Experts Back Off Claims

On Tuesday, two experts CBS hired to examine the documents told ABC News they could not vouch for them.

Knox said she believed the documents in question were fake, but also that they accurately reflected her boss's opinions.

CBS News on Tuesday said "CBS Evening News" anchor Dan Rather's (search) report on "60 Minutes II" last week did not rely on assessments made by the two examiners quoted in the ABC report, and found it notable that the secretary affirmed the content of the documents.

"We continue to believe in this story," said Betsy West, CBS News senior vice president.

Bush Memos (pdf)

The memos in question imply that then-Lt. George W. Bush was given special treatment in order to get into the Texas Air National Guard, and that once there, he failed to fulfill his Guard obligations.

CBS says the documents, purportedly written and signed by Killian, one of Bush's commanding officers, indicated Bush didn't follow orders to take a physical and that Killian was being pressured to "sugarcoat" Bush's performance ratings. Killian died in 1984.

Bush's father, future President George H.W. Bush, was a Republican congressman from Texas at the time. His grandfather, Prescott S. Bush, was a Republican senator from Connecticut from 1952 to 1963.

Document expert Jerry Richards, who worked for the FBI for 20 years and now has a private practice, examined the documents and had his own doubts about their authenticity.

"I can find nothing, no kind of mechanical equipment, typewriter, composer, etc., ... that would be readily available and at a reasonable price that would be able to do the things that would form a document such as the questioned ones here," Richards told FOX News. "Based on that, I'd say [there's] a very, very high probability that it was done on a modern-day word processor."

Joseph Newcomer (search), an expert in computer-based typesetting, agreed. He pointed out to FOX News that several characteristics of the "CYA" memo, in which Killian purportedly wrote that senior officers pressured him to "sugar coat" Bush's record, indicated that it was written long after the early 1970s, and probably even after 1995.

Newcomer performed an experiment, typing out the letter in a current version of Microsoft Word (search), printing it on a transparency and then laying it over the CBS document to note the differences and similarities.

"By the way, I've done this experiment with three of the memos — of printing them out holding them to the light — and finding out that they are identical with what Microsoft Word does," Newcomer said. He added that the technique does not work with other documents signed by Killian that are known to be legitimate.

[Click here to find out why Newcomer, who worked on electronic-typesetting techniques in the early 1970s, thinks the documents are forgeries.]

CBS: Our Sources Are Reliable

CBS said its story about Bush's Guard service relied on much more than the documents. But the controversy has raised credibility questions for the network news organization.

Emily Will, a documents examiner from North Carolina hired by CBS News, told The Associated Press that she had expressed to CBS producers her doubts about the handwriting in the documents and whether they 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.

"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 News that "I did not authenticate anything and I don't want it understood that I did."

James told The Associated Press late Tuesday that 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.

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 doesn't hold any document-verification credentials.

Bill Glennon, a "technical consultant" used by CBS News who insisted the superscripts and proportional spacing in the documents did in fact exist in the 1970s, is not a certified document analyst. His knowledge of the capabilities of IBM typewriters comes from working in the 1970s as a typewriter repairman.

West said Will did not contact the network the night before the report aired.

"I am not aware of any substantive objections raised," the CBS News senior vice president said. "She did not urge us to hold the story."

West said James told CBS News that she needed to know more about the documents before rendering any judgments. CBS contacted five document experts before the report aired and two since, and continues to report the story, the network said.

FOX News' Jim Mills, Liza Porteus, Kelly Wright and The Associated Press contributed to this report.