Federal prosecutors have begun interviewing current and former General Motors Co. GM employees as part of their criminal probe into the automaker's mishandling of faulty ignitions switches, according to people familiar with the matter, reported The Wall Street Journal.

Among those prosecutors have summoned to voluntary interviews at the U.S. Attorney's Office in downtown Manhattan are current and former employees singled out by a 325-page company-commissioned report prepared by attorney Anton Valukas earlier this month into why it took GM a decade to recall vehicles with a potentially deadly faulty ignition switch.

GM has turned over reams of documents to the government including thousands of emails between company officials relating to the ignition switch defect, according to people familiar with the case.

Such emails, which prosecutors will scour to determine who knew what and when about the problem, are likely to form the backbone of any potential criminal case, defense lawyers and former prosecutors said.

Separately, a group of state attorneys general, including New York Attorney General Eric Schneiderman and Florida Attorney General Pam Bondi, is investigating the delayed GM recalls, according to their spokesmen. More are expected to follow suit, said James Tierney, the former Maine attorney general and the director of the National State Attorneys General program at Columbia University.

GM had previously disclosed that it was aware of one state attorney general looking into the matter. A spokesman for GM said the company was cooperating with the U.S. Justice Department investigation but had no further comment.

The federal and state investigations are still in their early stages. Prosecutors have yet to tip their hands to defense attorneys as to which individuals they might focus on.

GM has said that it has dismissed at least 15 employees, including in-house lawyers and engineers, in connection with the issue, who expected to come under scrutiny by investigators. Defense lawyers said that one of those employees in particular, Raymond DeGiorgio, appears to be the most exposed to possible criminal liability, based on what's become public so far and the findings of the Valukas report.

The midlevel engineer in charge of the ignition switch, approved a sub-standard switch design and then quietly changed it without telling anyone after a flood of complaints from customers and other engineers at the auto maker. He later misled company officials about changing the product, hampering efforts to connect the flawed part to car accidents, Mr. Valukas concluded in his report.

Mr. DeGiorgio, contacted Friday, declined to comment on his involvement with the ignition switch recall.

The report by Mr. Valukas, himself a former federal prosecutor, has given prosecutors a substantial head start in their investigation, identifying key GM decision makers at critical points in the saga, providing thoroughly footnoted accounts by some of those decision makers, and laying out a general framework of what transpired inside the company since the faulty ignition switch first came to light.

"It's given prosecutors a preview of what people are going to say, it's an introduction to the names and issues, and what one former U.S. Attorney's take is on the case," said one person familiar with the investigation. "But they're clearly taking it with a grain of salt realizing it's prepared by lawyers, who are paid by the company."

Conducting internal probes has in recent years become a standard part of corporate criminal investigations. They allow a company under investigation to signal to the public and government investigators that the company is taking the allegations seriously, help assuage shareholders, and, if done artfully, can help shape a government investigation in ways the company desires.

Still, the Valukas Report stands out for its depth, detail, because it was done so early in the process, and because it was released to the public. By commissioning an in-depth, highly critical and public report into the issue up front, GM is taking a significantly different tack in dealing with the problem than did Toyota Motor Corp., which paid a $1.2 billion criminal settlement earlier this year stemming from its coverup of a safety flaw.

Toyota's aggressively defensive legal strategy frustrated prosecutors, whereas GM's more cooperative and contrite approach could lead to more lenient settlement terms, a former federal prosecutor said.

A spokeswoman for Toyota said the company fully cooperated with government investigators.

Still, prosecutors appear wary of parts of the GM report, which appeared to clear CEO Mary Barra and top executives of any responsibility, according to people familiar with prosecutors' thinking.

The Justice Department has faced growing criticism for how it handles corporate crime because it often charges just the company itself with a crime, while not charging any individuals or company officials.

After the Toyota settlement was announced, U.S. District Judge William H. Pauley was among those to criticize prosecutors. "I sincerely hope that this is not the end but rather the beginning to seek to hold those individuals responsible for making these decisions accountable," Judge Pauley said after approving the settlement in a court hearing.

With Toyota's top leadership in Japan, that would have been a challenging task in the Toyota case. That's not an issue with GM. And the two Assistant U.S. Attorneys who took the lead in the Toyota case—Bonnie Jonas and Sarah E. McCallum—are also taking the lead in the General Motors probe.

"We expect them to try to make cases against individuals this time," said one person familiar with the probe.

The so-called "proffer interviews" that prosecutors and FBI agents have begun conducting in the GM case are voluntary for those summoned.

In exchange for agreeing to speak with investigators, prosecutors will often proffer a very limited immunity deal for anything divulged in the interview. The interviews are commonly known as "queen for a day" interviews because people have that limited one-day window to receive favorable treatment from prosecutors.

Anyone who refuses to attend a voluntary interview can be subpoenaed to testify under oath before a grand jury. Those called to testify to a grand jury are not allowed to have their lawyers present in the room, making it a less palatable option in most cases.

For prosecutors, grand jury subpoenas are a method of last resort, but one that Manhattan prosecutors had to resort to during the probe into Toyota Motor Corp. when Toyota officials stonewalled interview requests, according to people familiar with the matter.

There are similarities between the Toyota case, which was a first of its kind federal criminal probe stemming from an auto safety issue, and the GM case. Defense lawyers are looking at that earlier case for clues as to where the government could be heading in its probe.

In that case, the Justice Department charged Toyota with wire fraud connected to misleading statements company officials made regarding the fatal car defec