Police said they were trying to ascertain whether the 24-year-old - the first Kenyan to win the Olympic gold in the marathon at Beijing in 2008 - had deliberately killed himself by jumping off the balcony at his home in Nyahururu, a town in the Rift Valley, nearly 100 miles northwest of the capital Nairobi, yesterday.
Jaspher Ombati, the regional police chief for the area, said Wanjiru appeared to have suffered internal injuries after the fall and was confirmed dead by doctors at a nearby hospital.
"I can confirm that Wanjiru is dead. It is not yet clear whether it was a suicide or if he jumped out of rage, or what caused him to fall to the ground," Ombati told Reuters.
"He jumped from his first floor balcony to the ground. He was bleeding from the nose and the mouth, and may have suffered internal injuries."
Ombati said police were investigating a possible love triangle that could have been behind the death of Wanjiru, a former winner of the Chicago and London marathons.
Ombati said the athlete's wife Triza Njeri had come home to find him in bed with another woman, had locked the couple in the bedroom and ran outside.
Wanjiru then leapt from the balcony, he said.
Ombati said Njeri and Wanjiru's female companion are assisting police in investigating the death.
Last December, Wanjiru was charged in court with threatening to kill his wife and illegal possession of an AK-47 assault rifle.
Njeri later withdrew her accusation of attempted murder against him in court, saying the couple had reconciled.
He also suffered minor injuries from a car crash in January when he swerved to avoid an oncoming truck, hit a pot hole and rolled his car.
Wanjiru defied the heat of Beijing in 2008 to triumph in an Olympic record time.
As a youngster, Wanjiru moved to Japan in 2002 to attend high school after winning a cross country selection trial in Kenya.
Wanjiru had been due to run in the Virgin London Marathon last month - a race he had won in 2009 - but withdrew a few weeks before the event because of injury.
March 2011: Wanjiru out of the London marathon – but Lel is in
December 2010: Olympic marathon champion charged with threatening to kill his wife
December 2010: Olympic champion Wanjiru returns to seek another Virgin London title