(CNN) — A storm that tore across Sioux Falls overnight was so treacherous, even weather forecasters had to pause their work and hunker down.
Now city officials are investigating why not all of the city’s outdoor warning sirens sounded like they should have.
The severe weather that struck South Dakota’s largest city spawned at least one tornado, the National Weather Service said.
“Obviously we had widespread damage throughout the entire city,” said Todd Heitkamp of the weather service’s Sioux Falls office. “Most of that damage was a result of probably 100-mph-plus winds. And some of the damage was a little bit more intense, and we’re suspecting that was a result of some brief, spin-up tornadoes.”
Eight people at the Avera Behavioral Health Center were injured in the storm, Sioux Falls Mayor Paul TenHaken told CNN.
The hospital suffered storm damage and was evacuated.
Across the city, at least 37 structures either collapsed or had integrity issues, the fire department said.
At 11:30 p.m. Tuesday night, employees at the local National Weather Service office paused and took shelter “due to the fact that our employees felt that their safety was being jeopardized,” Heitkamp said.
The office “shifted the responsibility of the warnings to our sister office up in Aberdeen. And then at about 11:35 we resumed those responsibilities.”
While residents sift through the debris Wednesday, the city is investigating what went wrong with the outdoor siren system.
“There was actually a breach of our protocol, in that not the entirety of the siren system was sounded,” the mayor said. “So we’re doing an internal investigation on why that happened, why the existing protocol was not followed.”
But Heitkamp stressed that the sirens are outdoor warning sirens, not tornado sirens, and shouldn’t be used as the only method of getting tornado alerts.
“Those sirens are not meant to warn people that are inside. They’re meant to warn people that are outdoors,” he said. “That’s why it’s important to have multiple methods to receive weather information.”
The mayor urged residents to stay off the roads, especially because of power outages throughout the city.
About 8,500 electricity customers were in the dark Wednesday morning, Xcel Energy said. That’s down from a peak of 25,000 customers without power immediately after the storm.