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AUSTIN, Texas — Amid ongoing widespread power outages during historic winter weather, the Electric Reliability Council of Texas, or ERCOT, is facing criticism as millions demand answers.

Bill Magness, the president and CEO of ERCOT — which manages the flow of electric power in the state — told Nexstar’s KXAN ERCOT’s primary function is to keep that balance, to ensure large collapses don’t happen.

“As hard as these outages are, they avoid a much more catastrophic situation,” he said.

As of Tuesday morning, over four million Texans are without power after planned rolling blackouts were never brought back online. Since then, customers have wondered why some areas have maintained power, while others have been in the dark and cold for days.

“We should be able to restore some customers this afternoon due to additional wind & solar output, & additional thermal generation that has told us they expect to become available,” the company said in a statement. “But, the amount we restore will depend on how much generation is actually able to come online.”

On Tuesday, Texas House Speaker Dade Phelan requested a hearing to review the factors that led to these power problems, saying Texas “must cut through the finger-pointing and hear directly from stakeholders.”

Mexico’s government-owned utility says its operations were left short as the winter storm in Texas froze natural gas pipelines. Mexico uses gas to generate about 60% of its power, compared to about 40% in the United States.

The Mexican utility says that by midday Monday it had restored power to about 65% of the 4.8 million customers affected by the blackout, mainly in the northern border states of Chihuahua, Coahuila, Nuevo Leon and Tamaulipas. A smaller number of users were also affected in Durango and Zacatecas.

The Federal Emergency Management Agency said in its Tuesday daily operations briefing that Texas officials have requested 60 generators from the agency and the priority for their use will be hospitals and nursing homes.

FEMA said in the briefing that 35 shelters with a total of more than 1,000 occupants have been opened in Texas.

The extreme cold is increasing energy demand at the same time the storm has reduced energy generation. FEMA officials said the lower energy output is due to a variety of factors, including icing on wind turbines and heavy cloud coverage that has reduced solar power generation.

What went wrong in Texas?

The extreme cold is increasing energy demand at the same time the storm has reduced energy generation. One of the major causes for the energy shortage has been the impact on natural gas, coal, and nuclear facilities.

Dan Woodfin, a senior director for the Electric Reliability Council of Texas, told Bloomberg frozen instruments at natural gas, coal, and nuclear facilities and limited supplies of natural gas is the main reason for widespread energy shortages.

“We’ve had some issues with pretty much every kind of generating capacity in the course of this multi-day event,” he told Bloomberg.

Some energy companies have begun implementing rolling power outages in response to overwhelming demand for power during record-setting low temperatures and wind chills. Rolling outages are expected to last at least through the end of the day on Tuesday.

The Southwest Power Pool, a group of utilities across 14 states, called for rolling outages because the supply of reserve energy had been exhausted. Some utilities said they were starting blackouts, while others urged customers to reduce power usage.

Evergy in eastern Kansas and western Missouri began 30 to 60-minute blackouts Monday shortly after noon. City Utilities in Springfield and the city of Independence also implemented rolling blackouts to their customers.

The power blackouts come as nearly all of Missouri is gripped by temperatures that barely reached zero degrees, and wind chills that in some areas registered at minus 30.

In Port Arthur, Texas, the nation’s largest oil refinery shut down on Monday because of the winter weather. Motiva said it shut down the refinery due to “unprecedented” freezing conditions along the Gulf Coast.

The company said it was monitoring the weather and would resume operations “as soon as it is safe to do so.”

Many power-generating plants in the state remained offline, causing utilities to impose rolling blackouts.

The Associated Press contributed to this report.