Police said Ellis' DNA was found on the bodies of nine women who were killed between 1986 and 2007 on the city's north side. Investigators believe eight of the women were prostitutes and one was a runaway. They ranged in age from 16 to 41.
Flynn said police used a warrant to obtain Ellis' DNA on Friday, so investigators had to move quickly to test the evidence because Ellis knew they were looking for him. He was arrested in nearby Franklin.
"Good police work and good police science led us to Walter Ellis," Flynn said. "These cases, some more than 20 years old, were never forgotten."
Online court records show that Ellis pleaded no contest in 1998 to a reduced charge of second-degree reckless injury. He was sentenced to five years in prison.
Flynn didn't know whether Ellis had an attorney. A message left with Ellis' previous attorney was not immediately returned Monday.
A brother of one of the victims said earlier Monday that his family has carried a great burden since his sister, Joyce Mims, was strangled in 1997 at the age of 41. Terry Williams said the family thinks the killer might have been related to his sister's boyfriend at the time. That man has since died.
"We just hated that it had taken so long for them to find her killer, those women's killer. But you know, justice one day is better than no justice at all," said Williams, 49, of Madison.
Before Monday, investigators had said they believed one person was responsible for seven deaths. Police now believe he's connected to the deaths of two more women who were both strangled.
Flynn and Chisholm announced a new investigation four months ago after DNA evidence linked seven deaths to one person.
Police said then that they never stopped investigating the cases, but scored a major breakthrough when DNA technology suggested the same person was responsible.
The investigation produced breaks in other cases after detectives resubmitted numerous DNA samples to the state crime lab in the unsolved homicides of suspected prostitutes. The work led to progress in at least 10 unrelated killings, authorities said.
"This is a phenomenal fete," said Mayor Tom Barrett. "The lesson here is there is never a totally cold case in the city of Milwaukee."