Four people were killed and about one million people were without power as intense thunderstorms swept through Texas on Thursday evening, bringing heavy rain, destructive winds and dangerous flooding to portions of the state that had already been inundated this month.

There were reports of blown-out windows, an office building wall caving in and downed power lines across Houston as a powerful storm tore through the downtown area. Four people were killed, officials said at a news conference. At least two people were killed by falling trees and one was killed in an accident involving a crane that was toppled over by strong winds, according to Samuel Peña, the city’s fire chief.

Much of Houston was battered by winds of speeds up to 100 miles per hour, rivaling speeds seen during Hurricane Ike, with “some twisters mixed in,” the city’s mayor, John Whitmire, said at the briefing. It will take at least 24 hours and up to 48 hours in parts of the city to restore power, he said.

The public school district in Houston said all schools would be closed Friday. Emergency responders warned residents to stay home, saying most traffic lights were out throughout the city and that fire fighters had to remove a live power line from a major highway.

Local news broadcasts reported considerable damage in downtown Houston, with metal sign posts twisted by the force of the winds and shattered glass.

Forecasters issued a string of flash flood warnings across the state earlier in the afternoon, warning Texans in those areas to seek higher ground and avoid driving through flooded roadways.

Images and videos circulating on social media emerging from east-central Texas on Thursday showed vehicles that appeared to struggle driving through flooded roads in College Station, Texas, which was under a flash flood warning through the evening.

One video posted in the evening showed strong winds whipping large panel structures at Minute Maid Park, where the Houston Astros were playing the Oakland Athletics.

The Weather Prediction Center said earlier Thursday that more than 12 million people across Texas, Louisiana and Mississippi faced the threat of excessive rainfall that could produce flash flooding and warned of potential heavy rains and flooding north of the Houston area on Thursday night.

Lina Hidalgo, the top executive of Harris County, which includes Houston, said earlier on social media that rain was expected to move through Harris County “fairly quickly” on Thursday night.

“But the worst case scenario is that heavy rain could hit the East Fork of the San Jacinto River, impacting residents and eventually causing more flooding as we get into the weekend,” she said.

Portions of Harris County, including areas near the San Jacinto River, had already been hit with major flooding earlier this month. The flooding prompted Ms. Hidalgo to issue a disaster declaration that would bring federal aid to Harris County residents who were affected by the storms.