The vertical spacing used in the memos, measured at 13 points, was not available in typewriters, and only became possible with the advent of computers.
CBS News stands by its report. "The documents are backed up not only by independent handwriting and forensic document experts but by sources familiar with their content," it said in a statement. "Contrary to some rumors, no internal investigation is under way at CBS News nor is one planned. We have complete confidence in our reporting and will continue to pursue the story."
The White House is declining to comment on the veracity of the documents. Many Democrats are worried that if they are found to be forgeries, it will be a setback for Sen. John Kerry's campaign to defeat Bush in November.