Confirming the incident, County Commissioner Joshua Nkanatha said the rescue operation has been called off.
Nkanatha said authorities are looking for the owner of the building, as investigations commence.
The authority said most of the collapsed buildings were constructed with substandard materials.
"Professional misconduct, using unqualified labour, greed for wealth by developers, poor structural design, and weak foundations are also to blame," said NCA chairman David Gaitho.