WhatsApp can just turn off encryption when they want, without users knowing:
http://heise.de/-2630361
That's probably what happened in June:
"Investigators said earlier they had detained 16 people in the anti-terror raids after working with U.S. authorities to monitor suspects' communications on WhatsApp Inc.'s messaging service."
http://www.bloomberg.com/news/articles/2015-06-08/belgium-ar...