GM spokesman Patrick Morrissey said Millikin was not forced out, and, in the past, had even been asked to stay on longer by Barra and the board. He declined to comment on any criminal investigation.
State and federal prosecutors are investigating GM for issues related to the faulty switch.
It is not clear whether prosecutors are developing any cases against individuals, as their main focus has been on building a straightforward mail and wire fraud case against GM, sources told Reuters in June.
As part of their attempt to build a case against the company, sources said in August that prosecutors are looking at whether lawyers who attended key meetings about GM's switch problems acted appropriately after the meetings or whether they mishandled information discussed.
Millikin's legal department came in for heavy criticism in the 325-page report by Valukas released by GM in June. The report said Millikin did not know details of the ignition switch problem until this year.