Patel Dam which burst, killing more than 40 people in Solai, Nakuru, was built illegally, the water authority has said.
The dam is one of a number on a sprawling farm near Solai, 40km from Nakuru town.
But none of them had a permit, a Water Resources Management Authority (Warma) spokesman said.
The farm's manager has denied any wrongdoing.
The search for dozens more missing people continues amid fears the death toll will rise.
The regional police commander announced it had reached 45 by Friday afternoon, after a teenage boy's body was found.
Police had earlier said almost half the victims found so far were children.
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An investigation into the tragedy has been announced, with a report into "cause and culpability" requested by the public prosecutor's office within two weeks.
It is likely to take a close look at the dam, which Warma spokeswoman Elizabeth Luvonga said lacked the documents needed.
"None of them have permits. That is why they are illegal," she told Reuters news agency.
But the general manager of the farm, Vinoj Kumar, denied the accusation.
"All these dams were built about 15 to 20 years before. There's no [nothing] illegal," he said.
The dam burst at about 9pm on Wednesday, sending a reported 70m litres of water down towards the homes below.
According to Daily Nation newspaper, the wall of water was about a metre-and-a-half high and 500m wide.
It destroyed everything in its path - including a primary school and power lines.
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