Nearly 1 million homes and business remained without power Friday from Texas to Louisiana after fierce thunderstorms swept hurricane-force winds through Houston, blowing windows out of downtown skyscrapers and toppling transmission towers.

At least four people were killed and officials warned that electricity outages for some residents could last days.

The widespread destruction brought much of Houston to a standstill as crews raced to restore power and remove uprooted trees and debris. School districts in the Houston area canceled classed for more than 400,000 students and government offices were closed. City officials urged people to stay off roads, many of which were flooded or lined with downed power lines and malfunctioning traffic lights.

Houston Mayor John Whitmire called downtown “a mess” and told people not to go to work Friday, unless they were considered essential workers.

“Stay home, take care of your children,” Whitmire said in a Thursday evening briefing. “Our first responders will be working around the clock.”

Whitmire said four people died during the severe weather. At least two of the deaths were caused by falling trees and another happened when a crane blew over in strong winds, officials said. Whitmire said wind speeds reached 100 mph (160 kph) “with some twisters.” Whitmire said the powerful gusts were reminiscent of 2008’s Hurricane Ike, which pounded the city.

Hundreds of windows were shattered at downtown hotels and office buildings, with glass littering the streets below. Fallen trees, power lines and broken glass have made some areas impassable, the city said. The state was sending Department of Public Safety officers to secure the area.

At Minute Maid Park, home of the Houston Astros, the retractable roof was closed due to the storm. But the wind was so powerful it still blew rain into the stadium. Puddles formed on the outfield warning track, but the game against the Oakland Athletics still was played.

Flights were briefly grounded at Houston’s two major airports. Sustained winds topping 60 mph (96 kph) were recorded at Bush Intercontinental Airport.

The power outage numbers were slowly improving Friday morning, down to about 786,000 in Texas and 121,000 in Louisiana, according to Poweroutage.us. At one point, Texas had more than 900,000 outages.

CenterPoint Energy warned customers in the Houston area to “be prepared for extended weather-related power outages.”

The problems extended to the city’s suburbs, with emergency officials in neighboring Montgomery County describing the damage to transmission lines as “catastrophic” and warning that power could be impacted for several days.

Heavy storms slammed the Houston area during the first week of May, leading to numerous high-water rescues, including some from the rooftops of flooded homes.