“We’re not going to renege on verbal agreements,” she said.

And in many ways, the new rules codify practices that were already in place at certain United States attorneys offices, including the office in Manhattan, which led the G.M. investigation. As such, the prosecutors most likely followed practices that mirror Ms. Yates’s memo when investigating the G.M. case.

The stumbling block to charging G.M. employees stemmed from a lack of evidence and a high legal burden, according to the people briefed on the matter. The wire fraud statute, which is typically used to prosecute auto cases, requires prosecutors to prove that someone intended to defraud, not just that the conduct was deceptive.

That burden stands in contrast to other industries, including food and pharmaceuticals, in which intent is not required to prove criminal wrongdoing. As The New York Times reported in July, the automotive industry spent nearly five decades beating back efforts to strengthen criminal penalties.

Still, prosecutors emphasize the importance of charging companies, even when it would be inappropriate to charge their employees, because such cases, prosecutors say, can act as a deterrent. For example, the case against Toyota was a warning to the automotive industry, which has been quicker to issue recalls ever since.

And the G.M. investigation, along with pressure from lawmakers, has helped spur a wide-ranging reform of safety practices in the auto industry. The changes unfolded not only among automakers but also within the agency charged with overseeing them, the National Highway Traffic Safety Administration.