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SIOUX FALLS, S.D. (AP) — Severe weather, including a probable tornado, struck South Dakota's largest city, leaving a trail of damaged buildings, downed power lines, fallen trees and some reports of injuries, officials said Wednesday.

Fire Chief Brad Goodroad said at an early morning news conference that at least 37 structures collapsed in Sioux Falls or have structural issues after the storm rolled through the city in the eastern part of the state just before midnight Tuesday. There were no reports of deaths.

Sioux Falls, with an estimated population of 187,200, is about 240 miles (386.kilometers) southwest of Minneapolis, Minnesota.

National Weather Service lead meteorologist Todd Heitkamp in Sioux Falls says surveyors will work to confirm whether a tornado caused the damage. Heitkamp says most damage likely was caused by strong straight-line winds.

Sioux Falls schools delayed classes Wednesday as crews worked to clear debris, and city officials in Sioux Falls urged people to stay off the streets until 8 a.m.

Mayor Paul TenHaken said the city was looking into a "breach of protocol" with its outdoor siren warning system. TenHaken said most of the sirens sounded in the southwest part of the city, where the most serious damage occurred, but the system is supposed to sound sirens citywide.

Among the buildings damaged was Avera Behavioral Health. The storm ripped off part of the hospital's roof and caused significant damage to the building's windows, according to Avera spokeswoman Michelle Pellman.

"Seven Behavioral Health patients have been transferred to the Avera Heart Hospital," Pellman said. "Only one patient suffered an injury and it was non-life threatening."

The Avera Heart Hospital also had damage to its lobby and windows, but all patients are safe, Pellman told the Argus Leader. She said people should stay away from the area because of flooding and debris concerns.

Other damaged businesses included an Advanced Auto Parts store where a wall collapsed. Kohl's and Best Buy lost part of their roofs and Pizza Ranch suffered heavy damage.

Xcel Energy says as many as 25,000 customers were without power at one point because of the damage, but that number dropped about 2,100 customers by 6:30 a.m.

The Red Cross opened a shelter at the Sioux Empire Fairgrounds' armory for people displaced by the storm.

The National Weather Service warned of possible severe thunderstorms Wednesday across the Plains and Upper Midwest, stretching from western Nebraska, Minnesota and Iowa to Wisconsin. The likeliest threat was in western Nebraska, and the weather service warned of possible flash flooding in the north-central part of the state.

The Associated Press