REE did not have enough thermal power stations switched on during peak hours of April 28 when the surge caused a chain reaction leading to the power outage, Spain's Energy Minister Sara Aagesen said on Tuesday.
"The system did not have sufficient dynamic voltage control capacity," Aagesen told a news briefing in Madrid.
REE "told us that they made their calculations and estimated that (switching on more thermal plants) was not necessary at this time. They only set it for the early hours of the day, not the central hours."
The blackout that lasted for several hours caused massive gridlock in cities and left thousands stranded on trains and in elevators across the Iberian peninsula.
An extensive report on the investigation, which will be made public later on Tuesday, concluded that some power plants that are required by law to regulate the grid's voltage failed to do so in the moments before the power outage.
Power plants "should have controlled voltage and, moreover, many of them were economically remunerated to do so. They did not absorb all the reactive power that was expected in a context of high voltages," she added.
Europe electricity grids' frequency or voltage is maintained at 50 Hertz to ensure stability.
Even slight deviations can lead to damage of equipment and infrastructure.
Keeping it stable requires electricity generation companies to adjust their output to match demand and the grid operator usually has tools to inform them when to do so by monitoring continually the status of the frequency and knowing when demand might be higher or lower.
A drop in frequency shows demand has exceeded supply and a rise shows supply is not matching demand.
The investigation found no evidence of a cyber attack, she added.
