"I think the interesting thing is the documents - where did they come from?" Russert told radio host Don Imus. "Somebody at CBS knows where the documents came from - and the credibility of that person, and the agenda of that person," he said.

Taken aback, Imus asked, "So they all know, in other words?" Russert replied bluntly, "Yeah."

The NBC newser said it was wrong for Mapes' boss Dan Rather not to reveal at the outset - if not the source's identity, at least the fact that he or she had partisan motives.

So why doesn't CBS News come clean now and reveal the identity of the person who forced Rather to retire from his anchor post and destroyed the credibility of its news division?

"They can't give up their source - they can't compromise their source," Russert insisted.

Editor's note: