By Brendan O'Brien
MILWAUKEE (Reuters) - Wisconsin Republican Governor Scott Walker on Friday signed a bill into law that prohibits prosecutors from using the state's secret investigation law to probe political crimes - a measure used to convict four of his aides and investigate his campaign.
In Wisconsin, prosecutors can use a so called "John Doe" proceeding law to call witnesses, request search warrants and offer immunity without probable cause that a crime has been committed.
Under the legislation Walker signed, prosecutors can no longer use the law to investigate cases of bribery or political misconduct. Instead, the law is limited to the investigation of certain crimes, such as ones involving violent felonies.
"We may never know if there are political crooks popping champagne corks today knowing that their crimes will now be very difficult to investigate and even harder to prosecute," said Democrat Rep. Peter Barca in a statement.
The new law also places a six-month time limit on secret probes in Wisconsin and limits the secrecy to judges, district attorneys, law enforcement officials and investigators and not to witnesses and suspects under investigation.
"These changes were desperately needed in Wisconsin and our state is better off now that it's law," Republican Assembly Speaker Robin Vos said in a statement.
The Republican-led state Senate and Assembly approved the measure along party lines during the last several days.
In June 2012, Walker became the first governor in U.S. history to survive a recall election.
Two months later, a John Doe investigation was launched into coordination between Wisconsin Club for Growth, conservative advocacy group and Walker's campaign and other recall campaigns involving other candidates.
The Wisconsin Supreme Court in July 2015 ruled that Walker's campaign and conservative groups did not break campaign finance laws during recall elections.
A John Doe investigation also produced convictions against six people, including four of Walker's aides, when he was Milwaukee County Executive from 2002 to 2010, on charges ranging from embezzlement to misconduct in office.
Walker, who withdrew from the U.S. presidential campaign last month, was not charged in either investigation.