"There's no way that I, as a document expert, can authenticate them," Marcel Matley said in a telephone interview from San Francisco. The main reason, he said, is that they are "copies" that are "far removed" from the originals.

Matley's comments came amid growing evidence challenging the authenticity of the documents aired Wednesday on CBS's "60 Minutes." The program was part of an investigation asserting that Bush benefited from political favoritism in getting out of commitments to the Texas Air National Guard. On last night's "CBS Evening News," anchor Dan Rather said again that the network "believes the documents are authentic."

A detailed comparison by The Washington Post of memos obtained by CBS News with authenticated documents on Bush's National Guard service reveals dozens of inconsistencies, ranging from conflicting military terminology to different word-processing techniques.

The analysis shows that half a dozen Killian memos released earlier by the military were written with a standard typewriter using different formatting techniques from those characteristic of computer-generated documents. CBS's Killian memos bear numerous signs that are more consistent with modern-day word-processing programs, particularly Microsoft Word.

"I am personally 100 percent sure that they are fake," said Joseph M. Newcomer, author of several books on Windows programming, who worked on electronic typesetting techniques in the early 1970s. Newcomer said he had produced virtually exact replicas of the CBS documents using Microsoft Word formatting and the Times New Roman font.

Newcomer drew an analogy with an art expert trying to determine whether a painting of unknown provenance was painted by Leonardo Da Vinci. "If I was looking for a Da Vinci, I would look for characteristic brush strokes," he said. "If I found something that was painted with a modern synthetic brush, I would know that I have a forgery."

Meanwhile, Laura Bush became the first person from the White House to say the documents are likely forgeries. "You know they are probably altered," she told Radio Iowa in Des Moines yesterday. "And they probably are forgeries, and I think that's terrible, really."

Citing confidentiality issues, CBS News has declined to reveal the source of the disputed documents -- which have been in the network's possession for more than a month -- or to explain how they came to light after more than three decades. Yesterday, USA Today said that it had independently obtained copies of the documents "from a person with knowledge of Texas Air National Guard operations" who declined to be named "for fear of retaliation."

It was unclear whether the same person supplied the documents to both media outlets. USA Today said it had obtained its copies of the CBS documents Wednesday night "soon after" the "60 Minutes" broadcast, as well as another two purported Killian memos that had not been made public.

A detailed examination of the CBS documents beside authenticated Killian memos and other documents generated by Bush's 147th Fighter Interceptor Group suggests at least three areas of difference that are difficult to reconcile:

• Word-processing techniques. Of more than 100 records made available by the 147th Group and the Texas Air National Guard, none used the proportional spacing techniques characteristic of the CBS documents. Nor did they use a superscripted "th" in expressions such as "147th Group" and or "111th Fighter Intercept Squadron."

In a CBS News broadcast Friday night rebutting allegations that the documents had been forged, Rather displayed an authenticated Bush document from 1968 that included a small "th" next to the numbers "111" as proof that Guard typewriters were capable of producing superscripts. In fact, say Newcomer and other experts, the document aired by CBS News does not contain a superscript, because the top of the "th" character is at the same level as the rest of the type. Superscripts rise above the level of the type.

• Factual problems. A CBS document purportedly from Killian ordering Bush to report for his annual physical, dated May 4, 1972, gives Bush's address as "5000 Longmont #8, Houston." This address was used for many years by Bush's father, George H.W. Bush. National Guard documents suggest that the younger Bush stopped using that address in 1970 when he moved into an apartment, and did not use it again until late 1973 or 1974, when he moved to Cambridge, Mass., to attend Harvard Business School.

One CBS memo cites pressure allegedly being put on Killian by "Staudt," a reference to Col. Walter B. "Buck" Staudt, one of Bush's early commanders. But the memo is dated Aug. 18, 1973, nearly a year and a half after Staudt retired from the Guard. Questioned about the discrepancy over the weekend, CBS officials said that Staudt was a "mythic figure" in the Guard who exercised influence from behind the scenes even after his retirement.

• Stylistic differences. To outsiders, how an officer wrote his name and rank or referred to his military unit may seem arcane and unimportant. Within the military, however, such details are regulated by rules and tradition, and can be of great significance. The CBS memos contain several stylistic examples at odds with standard Guard procedures, as reflected in authenticated documents.

In memos previously released by the Pentagon or the White House, Killian signed his rank "Lt Col" or "Lt Colonel, TexANG," in a single line after his name without periods. In the CBS memos, the "Lt Colonel" is on the next line, sometimes with a period but without the customary reference to TexANG, for Texas Air National Guard.

An ex-Guard commander, retired Col. Bobby W. Hodges, whom CBS originally cited as a key source in authenticating its documents, pointed to discrepancies in military abbreviations as evidence that the CBS memos are forgeries. The Guard, he said, never used the abbreviation "grp" for "group" or "OETR" for an officer evaluation review, as in the CBS documents. The correct terminology, he said, is "gp" and "OER."

In its broadcast last night, CBS News produced a new expert, Bill Glennon, an information technology consultant. He said that IBM electric typewriters in use in 1972 could produce superscripts and proportional spacing similar to those used in the disputed documents.

Any argument to the contrary is "an out-and-out lie," Glennon said in a telephone interview. But Glennon said he is not a document expert, could not vouch for the memos' authenticity and only examined them online because CBS did not give him copies when asked to visit the network's offices.

Thomas Phinney, program manager for fonts for the Adobe company in Seattle, which helped to develop the modern Times New Roman font, disputed Glennon's statement to CBS. He said "fairly extensive testing" had convinced him that the fonts and formatting used in the CBS documents could not have been produced by the most sophisticated IBM typewriters in use in 1972, including the Selectric and the Executive. He said the two systems used fonts of different widths.

On last night's "CBS Evening News," Rather said "60 Minutes" had done a "content analysis" of the memos and found, for example, that the date that Bush was suspended from flying -- Aug. 1, 1972 -- matched information in the documents. He also noted that USA Today had separately obtained another memo from 1972 in which Killian asked to be updated on Bush's flight certification status.

CBS executives have pointed to Matley as their lead expert on whether the memos are genuine, and included him in a "CBS Evening News" defense of the story Friday. Matley said he spent five to eight hours examining the memos. "I knew I could not prove them authentic just from my expertise," he said. "I can't say either way from my expertise, the narrow, narrow little field of my expertise."

In looking at the photocopies, he said, "I really felt we could not definitively say which font this is." But, he said, "I didn't see anything that would definitively tell me these are not authentic."

Asked about Matley's comments, CBS spokeswoman Sandy Genelius said: "In the end, the gist is that it's inconclusive. People are coming down on both sides, which is to be expected when you're dealing with copies of documents."

Questions about the CBS documents have grown to the point that they overshadow the allegations of favorable treatment toward Bush.

Prominent conservatives such as Rush Limbaugh are insisting the documents are forged. New York Times columnist William Safire said yesterday that CBS should agree to an independent investigation. Brent Bozell, president of the Media Research Center, called on the network to apologize, saying: "The CBS story is a hoax and a fraud, and a cheap and sloppy one at that. It boggles the mind that Dan Rather and CBS continue to defend it."

Staff reporters James V. Grimaldi and Mike Allen and researcher Alice Crites contributed to this report.