Do you know how/when they discovered it was not a prank...code words?
No, I don't. But on CNN's Late Edition this morning, Wolf Blitzer was interviewing Executive Douglas M. Duncan, Montgomery County Police Chief Charles Moose, FBI agent Gary Bald.
During the interview Blitzer was asking Moose why did the LE not do or recognize something and Moose politely with a smile ask Blitzer why CNN did not report to LE that the snippers had called them. Blitzer had to admit that they had CNN had racked it up as a prank call.