Attack on Titan 2nd season was announced and is in pre-production right now. The source is German though, because it was announced by anime producer George Wada during a German Q&A session in Bonn at the "animagiC 2014" event.
German Source:
http://www.crunchyroll.com/anime-new...titan-qa-panel
Relevant part:
"[...]Wada-san redete dabei nicht allzu lange um den heissen Brei herum und gab offen zu, dass sich eine zweite Staffel bereits in Vorproduktion befindet, was für tosenden Beifall im Saal sorgte. Er konnte jedoch keine weiteren Informationen geben und wies sämtliche plot-bezogenen Fragen im Folgenden mit der Aufforderung, doch bitte die zweite Staffel zu gucken, ab.
[...]"
Translation:
"[...]Wada-san did not prolong it much longer and openly admitted that a second season is in pre-production already, which caused the audience to cheer laudly. He could, however, not give any further information away and denied any plot-related questions, with the request, to kindly watch the second season.
[...]"