At least four people were killed and three injured in a bomb explosion on Saturday in northeast Nigeria's Maiduguri city, a security source and a witness told Xinhua news agency.The explosion occurred at about 7.10 a.m. when a tricycle attempted to speed by a bus along a major highway near the office of the state security service (SSS) in the city, Martins Ekpe, who was near the scene when the blast occurred, told Xinhua over phone."I was at the motor park to board a vehicle when the explosion occurred. The park is near the SSS office and a tricycle was targeting a bus that just left the motor park but the explosion happened before the tricycle could get to the bus. Four people, including the bomber, were killed," the source said.A resident named Ahmed Sheriff said he nearly ran into the blast."I was in another tricycle though in the opposite direction but was just about 500 metres from the spot of the blast," he said.A security source said he saw three bodies being evacuated from the scene about 20 minutes after the arrival of emergency and rescue workers.