At least one person was dead and more than 4 million were without power in Texas after a rare deep freeze forced the state’s electric grid operator to impose rotating blackouts because of higher power demand.

The cause of the death of the person was suspected to be exposure to “extreme low temperatures”, Houston Police Chief Art Acevedo said.

The PowerOutage.us website, which tracks power outages, said 4,113,701 Texas customers were experiencing outages at 2:05 a.m. ET (0705 GMT) on Tuesday.

The cold snap sweeping Texas reached the northern part of neighboring Mexico as well, where authorities said 4.7 million users lost power early on Monday. Around midday, service had been restored to almost 2.6 million of them.

President Joe Biden declared an emergency on Monday, unlocking federal assistance to Texas, where temperatures ranged from 28 to minus 8 degrees Fahrenheit (minus 2 to minus 22 Celsius).

“The Texas power grid has not been compromised. The ability of some companies that generate the power has been frozen,” Governor Greg Abbott wrote on Twitter. “They are working to get generation back on line.”

Abbott also deployed the National Guard statewide to assist in the restoration of electricity.

Houston’s George Bush Intercontinental Airport said it would remain closed until at least 1 p.m. CST (1900 GMT) on Tuesday, while the city’s Hobby Airport ceased operations until at least noon on Tuesday due to the inclement weather.

The freeze also took a toll on the state’s energy industry, by far the country’s largest crude producer, shutting oil refineries and forcing restrictions from natural gas pipeline operators.