hears what Russia had to say
https://www.rt.com/news/322393-russian-plane-crash-terrorist-attack/
hears what the USA had to say
http://www.cnn.com/2015/11/17/middleeast/russian-metrojet-crash-bomb/
an Arab source
https://english.alarabiya.net/en/News/middle-east/2015/11/18/ISIS-posts-photo-of-alleged-bomb-that-downed-Russian-jet.html
and a non-aliened source
http://www.ndtv.com/world-news/bomb-which-downed-russian-plane-was-hidden-in-soda-can-islamic-state-1244911?pfrom=home-lateststories
the point is they are all in agreement that this crash was caused by a bomb despite their vast political differences.
I hope this information is helpful

anti-aircraft guns surprisingly aren't terribly effective against modern aircraft. fighter planes and transport planes can usually fly high enough to stay out of range and attack helicopters are so heavily armored that the bullets usually just bounce off of them. anti-aircraft guns are however devastating against trucks light armored vehicles and massed infantry attacks.

reports say that even after the crash security officials at the airport where the plane took off from were taking bribes to let people through unchecked.

my first thought was a bomb in the checked luggage or cargo hold just like air India 182 or Pan Am 103 the circumstances of the 3 crashes are all eerily similar for more information on the older crashes see
https://en.wikipedia.org/wiki/Air_India_Flight_182 and
https://en.wikipedia.org/wiki/Pan_Am_Flight_103