Burglars accessed KCB Thika Branch on Sunday, November 19 and removed five containers of 50-real notes, with an estimated value of Sh50 million.
They disabled the bank's internal alarms and sensors and the burglary remained undiscovered until Monday, November 20.
Three months before the incident, the criminals rented a commercial property in Thika town and dug 120 meters beneath blocks of houses to a position beneath the bank.
The gang had renovated the property and put up a sign indicating it was a landscaping company selling both natural and artificial grass as well as other plants.
Neighbours described how they had seen van-loads of soil being removed daily, but understood this to be a normal activity of the business.
The tunnel, being roughly 70 cm (2.3 ft) square and running 4 meters (13 ft) beneath the surface, was well-constructed.
It was lined with wood and plastic and had its own lighting and air conditioning systems.
The gang broke through 1.1 meters (3.6 ft) of steel-reinforced concrete to enter the bank vault and completed their mission.
According to Kiambu County police boss Adiel Nyange, three people were arrested in connection to theft.