CBS would have wanted more than one source, so fooling CBS may have involved a coordinated action on the part of several individuals. For example, someone like Burkett could have reported to Mapes that Killian complained about pressure to help Bush, or that Bush had disobeyed an order to undergo a medical exam. A second collaborator might have sent the forged memos.
Or maybe Burkett faxed the memos while Burkett's information was simultaneously being spoon-fed to CBS through Democratic back channels. CBS, wishing to believe this negative story about Bush, might have treated the collaborating sources as corroborative. Then CBS on their own initiative might have assembled the other weak collaborating evidence such as the opinions of handwriting experts etc., (which weak corroborative evidence has subsequently collapsed).
There is supposedly a Yahoo board posting by Burkett claiming he gave some information to Cleland by telephone. Cleland according to Burkett was someone overtly wishing to be very aggressive in campaigning against Bush. Suppose Max Cleland went on to pass Burkett's unsubstantiated accusations on to others and the information worked its way back to Mapes through the Democrat grapevine.
On a more constructive note, IMHO the major news organizations would be doing us all a great service if they established improved internal review procedures for the information they report. Even though strategically this story probably plays for Bush in the near term, in the long run this sort of diversion from the political process really doesn't help anyone.
IIRC some source inside CBS said Mapes had been working on the story for 4-5 years, and they had people all over TX asking questions about Bush. Then, "like magic, this packet of documents showed up which answered all the questions".
If people don't see a fraud in that .. they are totally blind and have rocks for brains.