This topic has come up on other threads and we have never been able to come up with a case where this was actually done. I believe the consensus was LE would not use lying to the public as a tactic because of the issues of distrust it would cause. I believe one of our verified lawyers, Gitana, wrote about this issue and what she said made a lot of sense. I will see if I can find it.
Apologies if this has already been posted, but the use of DNA profiling was interesting to me at the time, and still is.
http://www.nytimes.com/2003/06/03/us/unusual-use-of-dna-aided-in-serial-killer-search.html