Four people were killed by a bomb explosion in Russia's volatile Chechen region in the latest attack against security forces in the North Caucasus, local prosecutors said on Sunday. Chechnya was engulfed by two wars since the break-up of the Soviet Union but has returned to relative calm in the last four years. Violence has become more frequent in nearby regions, notably Ingushetia, in 2008. In the latest attack, an explosion went off near an apartment building after police went to investigate an earlier report of gunfire around midnight near the capital Grozny. "Three people, who were police officers, and one passerby, who was a civilian, died in the explosion," the prosecutor said.