Complex sentence: For years, Umarov had been the top terrorist leader in Russia, and taken responsibility for several attacks on civilian targets since 2009, including the 2010 Moscow Metro bombings and the 2011 Domodedovo International Airport bombing.
Split into simpler sentences: For years, Umarov had been the top terrorist leader in Russia. He had taken responsibility for several attacks on civilian targets since 2009, including the 2010 Moscow Metro bombings and the 2011 Domodedovo International Airport bombing.