Officials say a frigid blast of winter weather across the United States is likely to blame for the deaths of two people in Texas as temperatures dropped into the single digits Fahrenheit (-22C), and snow shut down air travel and grocery stores.

More than four million people were without power Monday evening after surging demand was driven by people trying to keep their homes warm, and cold weather knocked some power stations offline and pushed Texas’s system beyond its limits.

After a facility storing 8,000 doses of the Moderna vaccine in Houston lost power, health officials hurriedly worked to distribute it before it expired at the same time authorities were pleading for people to stay home.

Texas law enforcement reported that two men were found dead along Houston-area roadways. Causes of death were pending, but officials said the subfreezing temperatures were likely to blame.

The toll of the worsening conditions included setbacks in the delivery of new COVID-19 vaccine shipments, which were expected to be delayed until at least midweek.

Several cities in the US saw record lows as Arctic air remained over the central part of the country. In Minnesota, the Chisholm-Hibbing weather station registered -39C (-38F), while Sioux Falls, South Dakota temperatures dropped to -32C (-26F).

In Kansas, where wind chills dropped to as low as -34C (-30F) in some areas, Governor Laura Kelly declared a state of disaster.

US President Joe Biden declared an emergency in Texas in a statement Sunday night. The declaration is intended to add federal aid to state and local response efforts.