A woman was taken out of the building screaming and said it was "a devastating loss", the report said.
The Middleton Police Department said in an alert that there was an "active shooter in the area".
Keely Arthur of CNN affiliate WISC said about 50 police cars and the Federal Bureau of Investigation are at the scene.
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"There is an actives hooter in the area, please lock all doors, stay inside, and shelter in place".
Greenway Station Mall and nearby schools were put on lockdown as a precaution.
Middleton is about 90 miles (145 kilometers) west of Milwaukee.
Evacuees are now being transferred to the Residence Inn by Marriott, Fox6 reported. Officials have asked people to avoid the area.
Emergency responders descended on the structure in Middleton, which houses Esker Software and WTS Paradigm, at around 10:30 a.m. following reports of a shooter there, according to the Wisconsin State Journal.