Wayne County Prosecutor Kym Worthy issued the subpoena last week, the Detroit Free Press reported.
Kilpatrick resigned as part of a plea bargain involving charges that he lied in a deposition about his romantic relationship with his former chief of staff, Christine Beatty, and about his role in getting police officers fired. Beatty is on trial in the case.
The former mayor also appears to be the target of a federal investigation and the city council ordered an audit of his office in January. Detroit's chief financial officer, Joseph Harris, charged last week that Kilpatrick and his aides had withheld credit card records from the auditor general.
Worthy would not comment on the investigation.
Text messages released Thursday in Beatty's trial revealed what seemed to be a pattern of free spending, with Kilpatrick's wife getting a leased Lincoln Navigator at city expense.