At least four persons sustained critical injuries when suspected militants exploded a powerful bomb in Indias north-eastern state of Manipur, police said Tuesday. Police said the militants exploded the bomb in Manipur capital Imphal near Bheirodan School in Khwairamband market complex, adding, the incident spot is very near to the residence of the regions Chief Minister Okram Ibobi Singh. The initial investigations have revealed that the militants detonated Improvised Explosive Device (IED), police said. \"However we are still investigating that whether bomb was planted or lobbed in the market place,\" police added. Earlier on August 15, a forceful explosion occurred just minutes before the celebrations of Independence Day in the state but no casualty was reported. Later on August 18 a powerful bomb blast left three persons injured. Several insurgent groups are active in Manipur. Some of them call for a separate state, others for regional autonomy while others demand complete independence from Indian rule.