The 52-year-old security officer was arrested Wednesday, but he has not been charged in the deaths.
The Town of Geneva Police Chief Steven Hurley said in a news release that the man was arrested in connection with the killing of Laura Simonson, 37, of Farmington, Minnesota, whose body was discovered June 5 in a suitcase discarded along a rural road in Town of Geneva.
Authorities have not identified the remains of the second female but say she was a white female with long dark hair, a pronounced overbite and a small heart tattoo on her lower left abdomen.
Investigators searched the man's West Allis apartment Wednesday and carried out objects in large, brown paper bags.
Simonson's father, Richard Wierson, told the Milwaukee Journal Sentinel that his daughter had placed escort ads on CraigsList. Wierson said she had struggled with mental illness since adolescence, and that her seven children were placed in foster care with him in 2010.
A missing persons investigation initiated by the Farmington Police Department determined Simonson was last seen Nov. 2 in Rochester.
The suspect is a licensed private security officer who has worked for Securitas Security Services USA 2007, according to the Wisconsin Department of Safety and Professional Services.
Town of Geneva is about 50 miles southwest of Milwaukee.