More than 20 DNA samples from other unsolved homicides are being re-sent to the state crime laboratory to check for possible links to the killer, police Chief Edward Flynn said at a news conference on Monday.
Most of the victims were known prostitutes. One woman was a drug abuser and seller.
On Monday police announced that the victims, killed between 1986 and 2007, were linked to the same DNA profile.
On Tuesday, police said they have linked another dead prostitute to the killer.
Flynn said the unknown killer has never been arrested for a felony, which is Wisconsin's basis for those who must submit to DNA testing.
The victims were Deborah Harris and Tanya Miller, who died in October 1986; Sheila Farrior, killed in 1995; Joyce Mims, killed in 1997; and Ouithreaun Stokes, murdered in April 2007.
The killer's DNA was also found on the body of 16-year-old Jessica Payne, a drug abuser killed in 1995.
CBS Affiliate WDJT correspondent Keith Meka reports that the police believe the man suspected in the slayings had sex with Payne and didn't kill her, but may know who did.
The Milwaukee Journal-Sentinel reports that the 7th victim tied by police to the same suspect is Florence McCormick, who was found dead in April 1995.
Flynn also said DNA tests showed the Milwaukee cases were not linked to murders of prostitutes that are part of active investigations in Los Angeles, Las Vegas and Mesa, Arizona.
Anyone working as a prostitute is in an "extraordinarily vulnerable population," Flynn said.
Milwaukee County District Attorney John Chisholm said improved technology makes it more likely the killer can be found. "We're convinced we're going to be able to bring justice to these victims and their families," Chisholm said.