According to the news outlet, the Middleton Police Department issued an alert shortly after advising people in area buildings to stay inside and lock the doors. He says he then saw numerous people running away from the building in "full sprint".
Images from the scene show armed police surrounding the building.
A State Journal reporter at the scene says at least 40 police squad cars and ambulances are outside the building.
University Hospital in nearby Madison says it's treating four patients from the shooting.
Wisconsin police are asking everyone near the scene of an reported active shooting to hunker down until further notice. There are stores and restaurants affected.
About 50 police cars plus the Federal Bureau of Investigation responded to the scene in the 1800 block of Deming Way, Keely Arthur of CNN affiliate WISC reported. The call came in around 10:25 a.m. on Wednesday, Sept. 19. She didn't have any other information.
Andrew King, an employee at TrafficCast, 1800 Deming Way, said he saw a man carrying a handgun inside the 1850 Deming Way building.
Middleton is about 90 miles (145 kilometers) west of Milwaukee.
Lahmers told the Associated Press she ran out of the building and hid behind a vehicle. We will update this story as soon as more information is available.