Police in Kitui County on Sunday arrested five suspects in connection with the theft of Sh1.5 million at a micro finance institution on Friday night.
The five are employees of Itoleka Financial Services Association's Mutune branch in Kitui Central.
The suspects are said to have dug a tunnel which they used to access the banking hall.
Also in custody is a night guard who was on duty on Friday. The guard alleged that he did not hear any commotion.
It is alleged that the bank employees decided to keep cash in their office desk drawers instead of using the bank's safes.
Detectives are also seeking to establish how the suspects managed to dig a tunnel and break into the banking hall unnoticed.
Kitui Central Deputy OCPD Edward Legei said the bank's night guard discovered the huge hole on the wall before he informed the police on Saturday morning.
"The night guard told the police that as he was patrolling the bank when he saw a tunnel," said Mr Legei who added that investigations had begun.
Police recovered the equipment used in digging the tunnel at the scene.