The Dane County Communications Center said police had received multiple calls about a gunman around the Deming Way block.
Judy Lahmers, a business analyst at WTS Paradigm, said she was working at her desk when she heard shots at about 10 a.m. She ran from the building and hit behind a vehicle.
She said the building's glass entrance door shattered.
Middleton is a suburb of Madison with about 17,000 residents.
A motive for the mass shooting is not yet known.
At least four people were injured in the shooting in Middleton, Wisconsin, on Wednesday, September 19, 2018.
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'I saw a dude walking with a gun inside the building, ' King said.
A total of five patients were taken to hospitals in nearby Madison following the shooting.
NBC 15 spoke to employees who work at a building next door who said they saw employees running from the neighboring building.
It is also unclear whether the shooter is still on the loose, although one unconfirmed report said they had been taken into custody. She didn't have any other information about the shooting, but said it was "totally unexpected".
"We knew at that point that something was going down".
Officers raced to the office building and set up a parameter around it.
Images from the scene show armed police surrounding the building.