BOGOTA, Colombia -- A massive blackout that left millions of Venezuelans without power has triggered one persistent question: How could a country with the world's largest proven oil reserves go dark?
The answer involves poor decision-making, rampant corruption, and the steady flight of electrical engineers, according to experts, though the government blames sabotage.
Here's a look at what led up to the historic blackout and why it has taken so long to restore power.
HOW DOES VENEZUELA'S POWER GRID WORK?
Venezuela's power grid relies heavily on the Guri Dam, a giant hydroelectric power station that was inaugurated in the late 1960s. While the engineers who planned Venezuela's current power grid imagined the Guri Dam would supply about 60 percent of the country's electricity, today it is believed to supply a whopping 80 percent. The remaining 20 percent comes predominately from thermoelectric power stations.
How much power Guri can actually supply and the level of Venezuela's current demand are not fully known because officials have not released that data in years. But engineers who closely examine Venezuela's power grid estimate demand has actually declined to around 10,500 megawatts — about the same levels as in 1999 — as industries have shut down and millions leave the nation.
Some engineers speculate Venezuela has even less supply than it did 20 years ago largely as a result of poor maintenance of existing plants.
WHAT WERE THE BIG CONCERNS BEFORE THE BLACKOUT?
For years engineers have been warning that Venezuela was sliding toward a prolonged blackout.
Critics say the Guri Dam on the Caroni River basin has long needed updates and become vulnerable during periods of drought.
The late President Hugo Chavez tried to remedy the country's over-reliance on hydroelectric power with what he called an "electricity revolution." Venezuela spent billions of dollars on new thermoelectric plants that in theory should be able to power the nation even without the Guri Dam, but they either were not built or were poorly maintained.
Military generals were put in charge of running Venezuela's power grid and qualified engineers — like many other professionals — began to leave the country.
The current head of both Venezuela's energy ministry and the state electricity company is a former National Guard major general with no apparent engineering expertise. On Twitter, he describes himself as "socialist, anti-imperialist and radically Chavista."
Chavez's successor, President Nicolas Maduro has responded to shortages by urging Venezuelans use less power by reducing the work week, and at one point even urged women to ration their use of hair dryers.
WHAT CAUSED THE BLACKOUT?
Maduro has blamed the blackout on a cyberattack on the plant's all-important electronic monitoring system, though engineers who have worked on the dam say they don't believe that.
One theory is that a fire knocked down one of the huge 765-kilovolt lines that connect the Guri Dam to Caracas. The regional electrical company that operated the dam decades ago had a fleet of helicopters that monitored the region for fires, but current workers say there is no functioning quick-response system in place. One worker, who spoke on condition of anonymity for fear of reprisal, said there has been a lack of routine maintenance, such as cutting down brush to keep it at a distance from the lines.
Though the three lines are spaced a considerable distance apart, Jose Aguilar, a Venezuelan energy expert who now resides in the United States, said a fire at just one of those could cause an outage affecting all of them.
Other theories include a breakdown in the electronic monitoring system that controls power distribution, a turbine going offline or a failure elsewhere.
"We don't know yet what happened because they're hiding it," Aguilar said.
COULD THERE ACTUALLY HAVE BEEN A CYBERATTACK?
Three engineers consulted by The Associated Press with direct knowledge of the Guri's operating systems say it is almost impossible. They explain that the computers that operate the monitoring system are not connected to the internet and can only communicate with each other, making them immune to an outside attack.
Anurag Srivastava, an engineering professor at Washington State University, said the only way to carry out a cyberattack in a closed system would be through physical access to the substation where the system is located.
Venezuelan authorities haven't provided proof of a cyberattack, but have pointed fingers at those they say are involved.
On the night of the outage, U.S. Sen. Marco Rubio tweeted about the outage minutes after it began, and Information Minister Jorge Rodriguez suggested he may have known about it in advance. On Tuesday, Venezuela's pro-Maduro chief prosecutor announced that opposition leader Juan Guaido is under investigation for the alleged sabotaged. The government has also detained a journalist who had previously spoken about Venezuela's vulnerabilities to a blackout, a video of which was shown on state television as proof he had prior knowledge of an attack.
HOW SOON WILL POWER BE RESTORED?
Venezuela's information minister said Tuesday that the power grid had been almost completely restored. But there are still pockets that have now gone six days in the dark.
Miguel Lara, a former general manager for the independent Venezuelan agency that collected data on electricity usage, said full power might be restored within days — or that problems could drag on for weeks. The worst-case scenario, he said, is an extended period in which power is restored in one place but then goes off hours later, or is restored in one city while another stays dark.
"I'm telling my friends to prepare for the worst," he said.
In the process of restoring the system, there have been multiple fires, including one at what was previously the country's largest transformer. Srivastava said fires at substations and transformers indicate engineers are using electrical loads higher than the equipment's rated capacity. Some engineers like Aguilar say that could point to a lack of expertise in how to put the system back together.
The country's heavy reliance on the Guri Dam could also complicate efforts, Srivastava said, because restoration depends on having access to multiple power sources. Dependence on one major power source is likely to slow down the recovery.
Soldiers were blocking access to the Guri Dam and adjacent power stations on Tuesday.
Associated Press writer Pableysa Ostos in Ciudad Guayana, Venezuela, contributed to this report.