The fourth major bombing in Russia in two weeks occurred Sept. 13 when an apartment building was blown up, killing at least 45 people. A similar bombing occurred Sept. 9, and some officials are blaming Islamic rebels battling for independence in the southern Russian region of Dagestan, where a bomb destroyed an apartment building for military families Sept. 4. The fourth explosion occurred at a shopping mall outside the Kremlin Aug. 31, killing one. There’s little defense against such attacks other than to watch for suspicious parcels and to follow local press reports about political tensions in the region.