MADISON, Wisconsin — A tornado struck northeastern Wisconsin on Monday, blackening the midday sky and sending workers sheltering in office basements as fierce winds splintered homes, scattered twisted metal across a highway and knocked out power to more than 30,000 people.

There were no reports of fatalities or injuries as of Monday evening, but emergency officials were still searching, said Gordon Hintz, Winnebago County’s top elected official.

The National Weather Service confirmed a tornado in Winnebago County, about 90 miles (145 kilometers) northwest of Milwaukee. Damage was reported in cities including Appleton and Menasha, where Police Chief Matt Albrecht said some homes and businesses were “severely impacted.”

The twister was spun off by a large storm system that moved through the Great Lakes, fueled by a buildup of heat and humidity, National Weather Service meteorologist Scott Berschback said.

Hintz said it could be several days before power in the stricken area is restored.

Heather Schroeder said she heard cracking and popping all around her as the tornado tore apart a laundromat, a bakery and a neighborhood tavern that were clustered around a street corner in Menasha, about 30 miles (50 kilometers) south of Green Bay.

Rescue workers pulled someone out of one of the buildings, said Schroeder, who works nearby. She said it wasn’t clear if the person had survived. Cars were turned upside-down, and there was a gas leak at the laundromat, she said.

“There’s nothing left,” Schroeder said.

The twister ripped roofs off homes and flipped vehicles in one neighborhood in Menasha, just blocks from the shores of Lake Winnebago. Video showed a trail of destruction, with some homes reduced to their outer walls.

Greg Van Driest was working in downtown Appleton when the sky got as dark as night.

“The word we all were using was ‘ominous,’” he said. “It’s like, ‘Whoa, look at that.’ ... You looked outside and you thought it was three in the morning, it was that dark.”

Hintz said he’s received multiple reports of buildings destroyed and vehicles overturned. All storm warnings that could be made ahead of time to get people to safety were activated, he said.

“Obviously when you look at the damage, Mother Nature doesn’t really care,” he said.

Weather service meteorologist Holley Kamba said the agency was receiving reports of damage in southern Outagamie County and eastern Winnebago County,

“We have been receiving calls of damage with a lot of trees down, power lines down,” Kamba said. “We’ve been searching for some pictures of the structural damage.”

Wisconsin Gov. Tony Evers said the state “is ready to assist communities however we can.”

Weather service meteorologists in Green Bay issued a tornado warning at 11:59 a.m. for the Fox Cities region, which includes Appleton, Menasha and Neenah. It warned of flying and dangerous debris, damage to mobile homes, vehicles and trees, while warning people to seek shelter.