Russia Alleges Ukrainian Intelligence Involvement in Deadly Concert Hall Attack

Over 140 killed in Moscow attack claimed by Islamic State; Ukraine denies accusations, calls them baseless.

Russian FSB claims Ukrainian military intelligence played a direct role in the attack.
Islamic State Khorasan has claimed responsibility for the March attack.
Over 20 suspects, including the alleged gunmen, have been arrested.
NATO accused of facilitating transfer of militants to Ukraine.
Ukraine dismisses allegations as Russian propaganda.