They said the sales numbers were inflated in part under pressure to preserve FCA's streak of U.S. monthly year-over-year sales increases, which now stands at 75 months. One source cited dealer complaints about the practice that reached CEO Sergio Marchionne before Bigland sought to end it. But he added that overstating of sales has crept back into play this year as competitive pressures on FCA's field staff have increased.

Meanwhile, the other company insider said the employee turnover rate among sales staff in the company's nine business centers around the country is four times higher than in the rest of the company because of pressure applied from above to meet aggressive sales targets.

FCA has declined to comment.

The revelations come as investigators from the Securities and Exchange Commission and the Department of Justice look into FCA's sales practices. One of the company sources said investigators visited all nine regional business centers beginning July 11 and spoke with current and former employees. Investigators also visited FCA field employees and past employees at their homes, the source said.

In January, FCA was accused by dealerships owned by Napleton Automotive Group, a Chicago-area dealership group, of civil racketeering in a lawsuit that alleged the automaker paid dealers to improperly inflate sales. FCA called the allegations "baseless" and sought dismissal of the suit. The lawsuit appeared to spur the federal investigation.

Bloomberg reported Monday, July 18, that the federal probe into FCA is in an early stage. The agencies would not comment on the scope or subject of their investigation.

FCA said it was "cooperating with an SEC investigation into the reporting of vehicle unit sales to end customers in the U.S." The company said that "inquiries into similar issues were recently made by the U.S. Department of Justice."

Both agencies and FCA declined to comment on whether warrants had been issued as part of the investigation.