Three people were seriously wounded Wednesday morning when a coworker opened fire at a software firm in a small Wisconsin city, police said.
The gunman, who was critically injured during an exchange of gunfire with law enforcement officers, died later at a hospital, Middleton police said.
The motive for the shooting, which unfolded in an office complex housing several businesses, is unclear.
Middleton Police Chief Charles Foulke said the gunman, whose name has not been released, was an employee of WTS Paradigm, a software firm where he began shooting about 10:30 a.m. Wednesday.
"It wasn't unusual for him to be in there," Foulke said. "He had a reason to be in there."
Foulke said the suspect exchanged gunfire with four law enforcement officers, whose actions he described as "very heroic ... and, from what I've been told and I believe, prevented much more bloodshed from happening."
"We have reason to believe the suspect was heavily armed with a lot of extra ammunition, a lot of extra magazines," the chief said.
A total of 10 police officers and sheriff's deputies initially responded and entered the two-building complex, Foulke said. They arrived as workers poured out of the officer complex.
"We know that timing is so critical," the chief said. "So any time there an active shooter in a target-rich environment, police officers go in, and that's what they did in this case."
According to Foulke, three victims were being treated for gunshot wounds at a local hospital. A fourth person suffered a graze wound not considered life threatening.
The shooting was being investigated with assistance from the FBI, the chief said.
About 50 police cars plus the FBI responded to the scene in the 1800 block of Deming Way, Keely Arthur of CNN affiliate WISC reported.
Middleton, a suburb of the state capital Madison, is a city of about 17,000 residents.
"My heart goes out to the people involved in the shooting," Middleton Mayor Gurdip Brar said. "You never would think this type of thing would happen in your city. Really, so sad that this happened here."
Update: This story has been updated to reflect new information from police regarding the number of victims injured.