Millions of homes in Texas were without power for a second straight day after historically cold winter weather caused a failure of the state’s electricity grid, triggering a public health emergency in the nation’s second most populous state.

Across Texas, temporary shelters filled up and fears rose that there could be an untold number of deaths from the historic cold. The grid operator said it continued to try to restart power plants hobbled by the weather, but it warned that full restoration could take days longer.

Gov. Greg Abbott called for an investigation into what caused a failure of the state’s power supplies, as between two million and three million Texans remained without electricity on Tuesday evening.