Three persons were injured when a powerful timer bomb exploded in the high-security office of the Manipur Public Works department in Imphal, capital of Manipur state, today. The bomb smashed window panes and damaged doors during peak office hour, pti reported quoting police as said. All the injured were admitted to hospital. The building has been heavily fortified by security men after government officials faced frequent extortion demands by various militant outfits. \'It was a very powerful timer bomb,\' said Additional Director General of Police V Jathang who along with DGF Y Joy Kumar visited the spot soon after the explosion