You might be interested in this set of videos[1] of public testimony by WhatsApp's Deputy General Counsel at a hearing on cybercrime in Brazil in December 2015, where he explains in some detail the type of information they have and don't have. He explicitly notes that they are incapable of accessing message content for end-to-end encrypted messages, and also explains that they don't retain copies of any communications once they've been delivered to recipients.

[1] http://www2.camara.leg.br/atividade-legislativa/comissoes/co...