From this Detroit Free Press report:

Fieger, a former Democratic candidate for governor, contends the probe was ordered by the Bush administration, which appointed Murphy as U.S. attorney in Detroit.
[U.S. District Judge] Borman said there are several unusual aspects about the case.
He said prosecutors failed to get Justice Department approval to conduct the investigation of campaign contributions until seven months after it started, a violation of department rules governing probes of campaign contributions.
He said prosecutors have admitted that this is the first prosecution of its type in eastern Michigan in at least 25 years. The Federal Election Commission normally handles such probes as civil case.