SAO PAULO (AFP) – Brazil on Wednesday sought to uncover the cause of a massive and mysterious blackout overnight, amid concerns of energy supply stability for the 2016 Olympic host nation.

The outage, which hit at 10:15 pm Tuesday (0015 GMT Wednesday) and lasted four to six hours, plunged nearly half the country into darkness.

An estimated 50 million people — more than a quarter of Brazil’s 190-million-strong population — were affected, mainly in the major cities in the southern part of the country, including Sao Paulo and Rio de Janeiro.

Off-duty police were called up as thousands were trapped in immobile elevators and subway trains. Cars were forced to nose through intersections made dangerous by suddenly extinguished traffic lights. Some cafes closed out of fear of a nocturnal crime wave.

Power was mostly returned before dawn, though several areas remained without energy, including nearly a quarter of the state of Minas Gerais, according to Globo television news.

Blame for the emergency was leveled at the Itaipu hydro-electric station that straddles the border between Brazil and Paraguay and supplies both countries with much of their energy needs.

Brazilian Energy Minister Edson Lobao said the problem originated there, possibly because a storm shorted out one of its five transmission lines.

Paraguay, which gets 90 percent of its power from the huge plant, was left in the dark for 15 minutes during the outage.

But Furnas, Brazil’s state-owned electricity company responsible for the affected area, said in a statement on Wednesday that it found no problem with the facility’s lines.

“Furnas’s transmission lines that link the Itaipu plant to the national interlinked grid are operating normally and no damage has been identified in its circuits and transmission towers,” it said.

It said that the National Electric Grid Operator “is seeking the causes of the blackout,” adding: “Any diagnostic at this time is purely speculative.”

The head of the Itaipu facility, Jorge Miguel Samek, told Globo he also believed a storm was likely behind the cut in power from the second-biggest hydro-electric plant in the world.

An energy ministry spokesman told AFP three of the plant’s transmission lines failed during the emergency.

Despite Furnas’s statement, the ministry’s secretary, Marcio Zimmerman, speculated that an unspecified “adverse meteorological condition” set off a “domino effect” through the grid.

Itaipu has an output of 14,000 megawatts, which supplies 20 percent of the energy needs of Brazil, Latin America’s most economically active nation. All of that was off-line overnight.

The blackout occurred two nights after the US television network CBS broadcast a report in which unidentified former US national security officials claimed massive power outages in Brazil in 2005 and 2007 were caused by cyber hackers attacking control systems.

Although Brazilian media were skeptical of that assessment, the US channel said those incidents should serve as a wake-up call to the United States, which could see its own power supplies hit by computer sabotage.

Brazil’s energy ministry was to hold an urgent meeting later Wednesday with all the country’s grid operators to determine the exact causes of the outage and what can be down to prevent a repetition.

Authorities are especially concerned about the damage that might be done to Brazil’s reputation ahead of Rio hosting the 2016 Olympic Games and the 2014 football World Cup.

The city, which is already struggling with rampant crime, was the worst-affected by the power cut, according to electricity grid officials.