The gunman also wounded four others attending the Living Church of God service on Saturday before shooting himself in the head.
“There is nothing significant in his background and no mental illness to indicate why he would do this,” Brookfield Police Captain Phil Horter told reporters.
Four victims died at the scene, while another three died later of their injuries. They ranged in age from teenagers to over-60 year olds.
Police have identified the gunman as 44-year-old Terry Ratzmann.
Church pastor Randy Gregory and his 17-year-old son James were among those killed.
Authorities said no suicide note was found during a search of Mr Ratzmann’s property.
Police said he attended the church, which met regularly at the Sheraton Hotel in Brookfield, a western suburb of the city of Milwaukee.
It was during a church gathering in a meeting room at the hotel that computer programmer Mr Ratzmann entered the room from the back, according to witnesses.
He fired a total of 22 shots into the crowd of about 50 to 60 people, police told reporters.
“There were so many shots. Everybody was screaming. It was mayhem,” witness Chandra Frazier was quoted as saying by the Milwaukee Journal Sentinel newspaper.
But police said the killer’s motive remains unclear.
Witnesses told authorities that a fortnight earlier he stormed out of a church meeting during a taped sermon by one of the church’s chief evangelists.
The evangelical church observes the Sabbath on Saturday, not Sunday.
Neighbours claim Mr Ratzmann was a devout churchgoer and avid gardener.
One neighbour was quoted as saying “he was the quietest guy in the world, the nicest.”