In a blow to its credibility, CBS News said today it had been misled over the authenticity of documents it aired in a story challenging US President George W. Bush's military service and announced it was mounting an internal investigation.
"Based on what we now know, CBS News cannot prove that the documents are authentic, which is the only acceptable journalistic standard to justify using them in a report," CBS News said in a statement.
"We should not have used them. That was a mistake, which we deeply regret."
The announcement marked a dramatic and embarrassing reversal by the network that just five days ago said it was satisfied with the accuracy of the documents first aired earlier this month in a "60 Minutes II" segment.
In a separate statement, the network's top anchor, Mr Dan Rather, apologized for what he called a "mistake in judgment" and said CBS News had been misled on the key question of how its source for the documents had obtained the papers.
The four memos, purportedly written and signed by the late Air National Guard Lt. Col. Jerry Killian, said that he was under pressure from his superiors to "sugar coat" Mr Bush's service record after Bush, then a Guard pilot, was grounded for his failure to perform to standards or to take a physical.
Immediately after the report was aired last Wednesday, Bush supporters and competing news organizations challenged the authenticity of the documents.
They said that comparisons of the Killian memos with other documents from Bush's National Guard service revealed inconsistencies in terminology and word processing techniques.