What killed the Mammoths
Disease? http://www.amnh.org/sciencebulletins/biobulletin/biobulletin/story981.html
Loss of habitat? http://biology.plosjournals.org/perlserv/?request=get-document&doi=10.1371%2Fjournal.pbio.0060099&ct=1
Climate change? http://www.worldhistoryblog.com/2006/05/humans-not-responsible-for-mammoth.html
A combination? http://artificialhabitat.wordpress.com/2008/04/01/what-killed-the-mammoths/
It is too early to tell for sure. More research is needed.