Storm Lake shook under a boom that could be heard and felt across the city Friday morning.
"We felt it here at the police station. It almost shook the building and we thought maybe it was a transformer blowing out right here," said Police Captain Bob Swanson later in the day.
A number of calls on the boom came into the county communications center, showing that the boom could be felt all over the community.
"We haven't been able to say for sure what it was, whether it was a sonic boom from some kind of jet, or something else," Swanson said.