The two suicide bombers also died in the conflagration Tuesday on a highway on the outskirts of Maiduguri, according to Deputy Superintendent Gideon Jubrin.
The civilians were passers-by hit by shrapnel when the cars crashed and exploded, said witness Abubakar Muhammed. Many other pedestrians and commuters in Tuesday morning traffic also were injured, he said.
The attack comes a day after Islamic extremist leader Abubakar Shekau threatened to unleash all-out war on Maiduguri, the birthplace of his Boko Haram terrorist network. He spoke in a video message.