Spanish grid operators ruled out cyber attacks, which was the reason for the massive cuts in parts of Spain, Portugal and France on Monday.

Eduardo Prieto, director of operations at Red Eléctrica, said preliminary findings showed that “the control system has no intervention” suggests an attack, echoing the previous day’s Prime Minister Luís Montenegro.

But the exact reason behind the cuts is unclear.

Grid operators said Tuesday they “can’t draw conclusions” until they get specific data. Spanish Prime Minister Pedro Sánchez said investigators are trying to find out the cause and then they will take all the necessary measures "to make sure this does not happen again."

Information about what happens when the cut is drawing information, leading to theories about the possible causes. Experts told the BBC that this could have been caused by several failures.

This is what we know, and what questions are still not answered.

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Sánchez said Monday night that the power of 15GW (equivalent to 60% of the demand at the time) was "suddenly lost from the system...in just five seconds".

Mr. Prieto said in a press conference on Tuesday that there was almost no "disconnection incident" in southwestern Spain, where there was a lot of solar power.

One issue that Spanish grid operators may have been referring to is that when the power company determines that the power supply mismatch can lead to unstable power and temporarily disconnect to protect its systems.

But Sanchez later said the power cut was "not a problem of too much renewable energy." He said the failure of no coverage - meaning supply - demand for electricity is relatively low, which is normal in crisis-to-crisis days.

Then what exactly happened? It is not clear, especially since many systems are powered at high frequency, not only renewable energy, but also disruptions of this scale occur on average about once a year.

A mismatch between power supply and demand can change the frequency of the grid, which is 50Hz in Europe and the UK.

If this frequency varies from a narrow range, it may cause damage to the device.

Professor Hannah Christensen at Oxford University said: “When a large company finds that frequency is out of tolerance, they can protect the device offline.”

She added that if many companies do this in a fast and continuous manner, it could have a "cascade effect" and lead to blackening.

However, when it comes to renewable energy, operators have very accurate short-term weather forecasts that predict when there will be solar surplus, so they adjust the power supply accordingly.

Renewables face different challenges in fossil fuel energy “due to its intermittentity,” but this is a well-known problem, she said.

"It's a little confusing, it won't be predicted," she said.

Professor Keith Bell of Strathclyde University added: “If the system relies on solar and wind, they designed a system that reflects this,” suggesting that the additional energy supply of renewable energy would not be surprising to the grid.

"Spain has a lot of wind and solar experience, as well as long-term forecasting of weather and its impact," he said.

"All systems will fail," he added. "Whether it comes from renewable energy, fossil fuels or nuclear power.

Red Eléctrica also suggested that the power drop results in a grid interconnection between Spain and France.

Two basic technologies are used for interconnecting grids or countries - a standard transmission line with alternating currents, a higher and higher high voltage DC line.

Professor Bell said there is a high voltage line in Spain that started using it seven years ago, which means it has been well tested.

The Iberian Peninsula is often called the "electricity island" because it relies on only a few links to France by the Pyrenees, which means it may be vulnerable to failure.

Sanchez said electricity was restored online due to links with France and Morocco and gas and hydropower sources.

Portugal's power grid operator Ren dismissed preliminary reports attributed to the agency on Monday, which said the power outage was caused by a rare atmospheric event.

The information in Portuguese says: "There is anomalous oscillation in very high voltage lines (400 kV) due to extreme temperature changes in internal or Spanish, a phenomenon called "inducing atmospheric vibration".

“These oscillations lead to synchronous failures between electrical systems, resulting in continuous interference from connected European networks.”

But Ren spokesman Bruno Silva told AFP on Tuesday that the grid operator "had not made this statement" without providing more details.