Copyright laws and public information (see below).

I am an author compiling stories for an inspirational book, featuring recounts from victims and witnesses of the Aurora, Colorado theater shooting, in their own words. Each chapter is a different story, crediting the specific author. I have received permission from all authors thus far, but need to know if online blog posts or specific statements posted on Facebook, online, or on camera for news interviews are allowed to be used to compile stories of people I have not been successful in contacting, but have an incredible story I''d like to share. If I use their quotes, or postings, do I have to get permission? Or can I simply credit the posting or interview? And do I need something in writing from each person, granting permission in order to avoid future risk/liability?