Police said the extent of the victims’ injuries was unknown.
Wauwatosa Police chief Barry Weber gave no motive for the attack at the Mayfair Mall in a brief update about three hours after the 2.50pm local time incident near an entrance to the Macy’s store.
He said the extent of the eight victims’ injuries was unknown, but all were alive. He added that the gunman was “no longer at the scene” when authorities arrived.
“Preliminary statements from witnesses indicate that the shooter is a white male in his 20s or 30s,” Mr Weber told reporters.
“Investigators are working on determining the identity of that suspect.”
The chief called the mall an active crime scene and asked people to continue to stay away. He said the mall would remain closed until further notice.
Witnesses told WISN-TV that they had heard what they believed to be eight to 12 gunshots.
Some people remained in the mall while police searched for a suspect. The station interviewed several people outside the mall who said they had friends sheltered inside stores while the search was ongoing.
Jill Wooley was inside Macy’s with her 79-year-old mother when they heard eight to 12 gunshots just outside the store entrance.
“We heard the first shot fired and knew immediately it was a gunshot,” Ms Wooley told CBS58 in Milwaukee. “We both just dropped to the floor.”
Ms Wooley said she did not see anyone but the shots were “very close”. She added that they ran in the opposite direction to the basement of the store, where they then hid.
Trish Cox’s 19-year-old nephew works at Finish Line sporting goods store.
She said she was worried because the store’s phone was not being answered and was frantic as she waited while FBI agents cleared the mall.
An agent who would not give his name said the mall was being “methodically” cleared. Heavily armed FBI personnel were visible at the mall.
Mall operator Brookfield Properties said in a statement they were “disheartened and angered that our guests and tenants were subject to this violent incident today”. They declined further comment.