Police had previously said at least two people were killed and four were wounded during the afternoon shootings at Accent Signage Systems, which is in a largely residential area on the city's north side.
"We do have several victims inside that are dead," Minneapolis Police Deputy Chief Kris Arneson said during an evening news conference two blocks from the company. But when pressed to say how many, Arneson refused and said police were still investigating.
She also wouldn't release details about the victims, but said the shooter's body was found inside the building.
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 received a 911 call around 4:30 p.m. from inside the business from someone reporting that there was a shooting.
Dozens of squad cars and police vehicles were still surrounding the business in the Bryn Mawr neighborhood by Thursday evening. 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."