Three men and a woman died on the spot - including the two drivers of a Toyota Premio and a Subaru Forester, both metallic grey. A fifth victim died in hospital.
The ex-Mungiki leader was travelling in the Subaru Forester, which rolled several times after going off the road when it was shot at.
A woman in the same vehicle with Mr Njenga died from the rolling impact since she was not shot at, police at the scene said.
The motives still unknown and the story is developing. MainaNjenga is currently recuperating at Aga Khan hospital.
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