COLOMBO, Sri Lanka — Sri Lanka’s president gave the military sweeping police powers starting Tuesday in the wake of the Easter bombings that killed nearly 300 people, while officials disclosed that intelligence agencies had warned weeks ago of the possibility of an attack by the radical Muslim group blamed for the bloodshed.

The suicide bombings struck three churches and three luxury hotels Sunday in the island nation’s deadliest violence since a devastating civil war ended in 2009. The government shut down some social media, armed security forces patrolled the largely deserted, central streets in the capital of Colombo, and a curfew went into effect.

The military was given a wider berth to detain and arrest suspects — powers that were used during the civil war but withdrawn when it ended.

Prime Minister Ranil Wickremesinghe said he feared the massacre could unleash instability and he vowed to “vest all necessary powers with the defense forces” to act against those responsible.

Adding to the tension, three unexploded bombs blew up Monday inside a van parked near one of the stricken churches as police were trying to defuse them, sending pedestrians fleeing in panic.