Updated

The authenticity of memos concerning President Bush's service in the National Guard during the Vietnam War era continue to create a swirl of controversy.

The Washington Post reported that documents CBS says were written by Lt. Col. Jerry Killian (search) — who, at the time, was Bush's commanding officer — bore markings showing they were faxed to CBS News from a Kinko's copy shop in Abilene, Texas.

Those documents, which surfaced last week, have raised questions about whether Bush shirked his military duties with the Texas Air National Guard. Several typography analysts have said that the documents are forgeries that were prepared on a modern computer rather than a typewriter in 1972 and 1973.

Robert Strong (search), one of three people interviewed by CBS' "60 Minutes," told the Post Thursday he was shown copies of the documents by CBS anchor Dan Rather and producer Mary Mapes on Sept. 5, three days before the broadcast that revealed the memos. He said at least one of the documents bore the faxed header "Kinko's Abilene."

The sole Kinko's in Abilene is 21 miles from Baird, Texas. Baird is the home of retired Texas National Guard officer Bill Burkett (search), who has been named by several news outlets as a possible source for the documents.

CBS so far has refused to reveal the identity of the source who provided the documents, even though many document experts have questioned their authenticity.

Bush Memos (pdf)

When asked what role Burkett may have played in CBS's reporting, CBS News President Andrew Heyward told The Washington Post: "I'm not going to get into any discussion of who the sources are."

Meanwhile, a newly surfaced document purportedly from Bush's military file — whose authenticity has not been verified — shows more "proof" that Bush failed to fulfill his military obligations more than 30 years ago, Salon.com reported Thursday.

In a 1968 contract posted on a Web site run by Philadelphia researcher Paul Lukasiak, Bush agreed to serve as a pilot for five years. The Web site that posted the document calls itself "The AWOL Project: An Examination of the Bush Military Files."

"I desire to enter undergraduate pilot training as an officer of the Air National Guard and, in consideration therefore (including the expenditure of extensive public funds), do hereby agree that upon successful completion thereof I will serve with my parent ANG unit as directed by the unit commander, unless sooner relieved by competent military authority, for a minimum period of five years," the agreement says.

Bush completed his initial training at Moody Air Force Base in Georgia on Nov. 26, 1969, and his final F-102 training on June 20, 1970. He received a promotion later that year and continued to participate in drills at Ellington Air Force Base in Houston.

On Sept. 6, 1972, Bush's request for a three-month transfer to 187th TAC Recon Group, Montgomery, Ala., was approved so he could work as political director for the Senate campaign of Winton M. Blount, a friend of his father. In November, he returned to Ellington to participate in drills, which he did through 1973.

On Sept. 18, 1973, Bush got permission to transfer to reserve status and was placed on inactive guard duty about six months before his six-year commitment was to end. He attended Harvard Business School in the fall. On Oct. 1, 1973, he received an honorable discharge.

Rather: 'We Will Keep an Open Mind'

Thursday's developments come one day after CBS News said once again that it stands by the basic content of its report.

But the network acknowledged 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."

"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," Rather said on "60 Minutes."

Later, Rather told The Washington Post: "If the documents are not what we were led to believe, I'd like to break that story …Any time I'm wrong. I want to be right out front and say, 'Folks, this is what went wrong and how it went wrong."'

Rather later told USA Today that no one has disputed "the heart" of his original report last week, but complained that a "thick partisan fogging machine seeks to cloud the core truth of our story by raising questions about the messenger, methods and techniques."

The memos in question indicate that Killian had been pressured to sugar coat Bush's performance and that the future president had ignored an order to take a physical.

CBS on Wednesday flew Killian's former secretary, Marian Carr Knox, 86, from Texas to New York for an interview, where she said that while 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."

When asked about Knox's comments, White House spokesman Scott McClellan said: "We don't have to rely on the feelings of a nice woman who has firmly stated her opposition to the president."

McClellan told reporters aboard Air Force One, "CBS has now acknowledged that the crux of their story may have been based on forged documents."

"There continue to be questions raised about these documents," he said as Bush flew to Minnesota to campaign Thursday.

As for Rather's comments, McClellan said, "so now some [journalists] are working on feelings instead of facts."

"It's always best for journalists to stick to the facts and not dispense campaign advice," McClellan added.

On Wednesday, 40 GOP House members circulated a memo calling on 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.

A House memo being circulated by GOP Whip Roy Blunt of Missouri calls on 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.

Rep. Chris Cox, 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."

FOX News' Liza Porteus and The Associated Press contributed to this report.