"He fell from the balcony of his home after a row with his wife. We found him lying on the verandah with a broken skull," local police chief Jasper Ombati said.
"The theory we are getting from his wife and a watchman is that he came home drunk from the bar and he was accompanied with a woman. When his wife asked why he came home with a woman, an argument ensued, and that is when he fell off the balcony and died," Ombati said.
"He was pronounced dead at the Nyahururu district hospital," he added.
He said police were also investigating whether the runner may have jumped rather than fell from the balcony.
"There is also another theory that during the argument, the wife ran away from their bedroom and as her husband ran after her, she locked the stair-case door prompting Wanjiru to look for an alternative way out... and that is when he jumped off the balcony to the ground and died," he said.
"These are the two theories we are investigating. We cannot conclusively say he committed suicide, or that he jumped down and died accidentally. All these are conclusions we will make once the investigation is completed," the area police chief said.
"For now we are going on with the investigations. Wanjiru’s wife has recorded a statement with us and their watchman as well. The woman Wanjiru went home with is at the station with us, assisting detectives with the investigations," he added.
Wanjiru, who over the past few months appears to have had marital problems, was involved in a serious car crash in January.
He was scheduled to appear before a Nyahururu court next Monday, accused of being in possession of an unlicensed firearm.