The shooting at Accent Signage Systems, in a largely residential area of the city, resulted in "a couple of fatalities" and at least four people being taken to hospitals, Minneapolis police spokesman Steve McCarty said.
Hennepin County Medical Center was treating three people from the scene, all in critical condition, spokeswoman Christine Hill said. She said the hospital wasn't expecting more patients with critical injuries.
Officers wouldn't release details about the shooter, including if he or she were among the victims. By Thursday evening, officers were slowly walking around the scene without their weapons drawn and interviewing neighbors.
Dozens of squad cars and police vehicles still surrounded the business, in the Bryn Mawr neighborhood on the city's north side. Traffic was stopped on a nearby bridge along Penn Avenue, where earlier in the day law enforcement officers had rifles drawn and pointed at a park below.
People from the neighborhood milled around but deputies kept them back.
Marques Jones, 18, of Minneapolis, said he was outside a building down the street having his picture taken when he and his photographer heard gunfire that sounded close.
"We heard about four to five gunshots," Jones said. "We were shocked at what happened and we just looked at each other. We all just took off running to our vehicles."
Accent Signage Systems' website says the company makes interior signage and listed its founder as Reuven Rahamim. A phone message left at the business and at a residential listing for Rahamim was not immediately returned.
"Very sad situation in Bryn Mawr," Mayor R.T. Rybak tweeted Thursday afternoon. "Please stay away and let the police do their work."
Hennepin County sheriff's spokeswoman Lisa Kiava said earlier that deputies had been called to the shooting, though she didn't have further information.