The bank, Evrofinance Mosnarbank, is based in Moscow and is jointly controlled by Venezuelan and Russian state-owned companies. The Treasury Department said that the bank’s assets grew by more than 50 percent last year, as the United States escalated its sanctions against Venezuela and as European financial institutions severed ties with the country.

Energy experts, Venezuelan power sector contractors and current and former Corpoelec employees have dismissed accusations of sabotage, saying the blackout was the result of years of underinvestment, corruption and brain drain.

The San Geronimo B substation connects eight out of Venezuela’s 10 largest cities to the Guri hydropower plant via one of the longest high-voltage lines in the world.

When visited on Sunday, the substation’s usual buzz of high-voltage cross currents had been replaced by total silence. A cow roamed amid the transformers. Several National Guard soldiers and a unit of police commandos were at the substation, but no employees were there.

The substation is vital “to supply the country in a stable way,” said José Aguilar, a Venezuelan power industry expert based in Chicago. Its paralysis means power is unlikely to be restored nationally until Tuesday at the earliest, he said.

The government declared Monday a holiday for schools and public workers.

What caused the blackout has been a source of speculation. A Corpoelec union leader, Ali Briceño, told reporters on Friday that a brush fire under a power trunk line destabilized the grid and caused Guri’s turbines to shut down. The government has struggled to restart the turbines since, he said.