If they have WMD evidence, revealing that hardly requires revealing sources. The only reason one would need reveal sources is if the "evidence" is nothing but hearsay..
By revealing what you found, it is possible that that would let reasonably intelligent figure out how you found out. If they figure that out, they can move or destroy additional material that the source they suspect would also know about.
In addition, if your source is a line of communication that the bad guys believe to be secure, you run the risk of alerting them that it is not.
"If they have WMD evidence, revealing that hardly requires revealing sources." No, in fact good security is set up so that when info is revealed by the other side it points directly to a very small number of sources. This is the reason behind much of the effort to keep info on NEED TO KNOW!