Nissan Motor Co. is suspected of fabricating documents at factories in Japan to make them appear as if certified inspectors had carried out final inspections on vehicles coming off the assembly line, sources close to the matter said Wednesday.

A government probe found the Japanese automaker had used uncertified staff to check vehicles at all six of its domestic factories, prompting Nissan to announce a recall of more than 1.2 million cars in Japan manufactured since October 2014.

If the fabrication of paperwork did happen, it would mean workers at the factories had intentionally sought to cover up the practice, rather than overlooked regulations on inspectors' qualifications. Final vehicle checks usually involve eight inspectors.

Officials at the Ministry of Land, Infrastructure, Transport and Tourism first inspected a Nissan plant in Kanagawa Prefecture on Sept. 18, triggering an internal probe by the automaker.

All six factories have now been examined by ministry officials, with the final two plants in Tochigi and Kyoto prefectures having been investigated on Tuesday.

Nissan is scheduled to report to the transport ministry its findings following the internal probe and measures to prevent similar practices in late October.