(CNN) - 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.
His identity has not been released. The suspect, a US citizen in his 40s, did not have any known criminal record or family who lived in the area, Chief Charles Foulke said Thursday.
Authorities searched the suspect's residence on the southwest side of Madison on Wednesday night. It's unknown if the gun used in the shooting was owned legally. But police said he did not have a Wisconsin conceal-and-carry permit.
The Bureau of Alcohol, Tobacco, Firearms and Explosives is investigating where the weapon was originally purchased.
"A significant number of bullets were fired" between the suspect and responding law enforcement, Foulke 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 was an employee of WTS Paradigm, a software firm where he began shooting about 10:30 a.m. Wednesday. The suspect had been employed at the company since April 2017.
"It wasn't unusual for him to be in there," Foulke said. "He had a reason to be in there."
During the shooting, people hid under desks or barricaded themselves, and many ran, he said.
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." The officers have all been placed on administrative leave, following department policy.
"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.
According to Foulke, three victims, two males and a female, are being treated for gunshot wounds at UWHealth, a local hospital. A fourth person was grazed.
A tweet Wednesday night from UWHealth said one person was in critical condition and the other two were in serious condition. The victim in critical condition had been upgraded on Thursday, Foulke said, declining to identify the victims.
The FBI is assisting in the investigation, 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. Police video is being reviewed, Foulke said. The building also has surveillance cameras.
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."
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