A roadside bomb ripped through a Pakistan Navy bus on a busy thoroughfare in the southern port city of Karachi on Thursday, killing two persons and injuring 15 others. This is the third such attack in two days. The bus was targeted near a naval facility on Shara-e-Faisalroad,officials said. Deputy Inspector General of Police Iftikhar Tarar said the attack was similar to the bombing of two Pakistan Navy buses on Tuesday that killed four persons and injured 56 others. Explaining the possible motive of the attack, Tarar said security agencies are the “main target” of terrorists. Two persons were killed and 15 injured by the blast, state-run PTV reported. At least three vehicles, including a bus, and a petrol pump were damaged by the explosion. The attack occurred at the same spot where slain former Premier Benazir Bhutto's motorcade was attacked by two suicide bombers shortly after she returned to Pakistan from self-exile in October 2007. No group has claimed responsibility for today's attack.