Nissan Motor Co. President Hiroto Saikawa on Friday blamed a lack of communication between workers and managers as one of root causes for conducting improper checks of vehicles at the company’s assembly plants using unqualified workers, in a practice that had been routine for more than 30 years.

Nissan said the first known occurrence of problematic inspections appears to date back to 1979 at its factory in Tochigi Prefecture. No problems concerning inspections were found at its Kyoto plant, it said.

Earlier in the day, the company submitted an investigation report to the transport ministry, which also cited a shortage of certified inspectors and poor awareness of compliance as reasons why unqualified workers had performed inspections of finished cars.

“We deeply apologize for betraying the trust of people and the transport ministry,” Saikawa said at a news conference at the carmaker’s headquarters in Yokohama.

Saikawa said he and other executives will return a portion of their salaries to March to take responsibility for the scandal, but declined to make public the exact amount.

The company said the distance between managers and workers had made it difficult to identify the improper practices. Saikawa lamented problem practices on the factory floor had not been fully identified by executives, including by his predecessor — and now Nissan chairman — Carlos Ghosn.

Saikawa said this was partly due to Nissan’s own culture of respecting the factory workers’ autonomy in solving issues and achieving the goals set by executives.

The company admitted some employees had been aware of the misconduct. But its internal whistleblowing system had not worked properly as employees “believed the issue would not be resolved even if they reported it.”

At some plants, trainees taking an exam to become certified inspectors were provided with the correct answers, the carmaker said.

The report said a shortage of certified inspectors at assembly plants also aggravated the problem. The company admitted it was not careful enough about securing the necessary numbers of qualified workers to carry out the inspection process.

Many of the inspectors were aware that quality checks performed by unauthorized trainees were against the government regulation, but they failed to maintain a conscientious approach to compliance for their work which the company conducts on behalf of the government, it said.

Nissan said it will implement a number of measures, including installing facial recognition security gates at plants by next March, to make sure only certified inspectors can enter the area to carry out inspections on finished cars.

It also plans to increase the number of certified inspectors by 80 or more, it said.

“I believe the only way to regain customer trust is to show our concrete efforts through actions,” Saikawa said.

The revelation first came to light in September when on-site investigations by transport ministry officials found unqualified workers were involved in the final inspections of finished cars at Nissan’s domestic plants.

The unauthorized safety inspections continued until mid-October even after Saikawa’s apology at a news conference on Oct. 2, prompting its decision to suspend shipment and production of vehicles for the domestic market. However, operations at all six Nissan factories have been back to normal since Nov. 7.

Nissan initially planned to file the report with the transport ministry by the end of October, but delayed the submission in the wake of a spate of irregularities.

Nissan has recalled about 1.2 million vehicles in Japan that went through flawed inspections. Vehicles for export are not subject to the recall as the company says the regulations violated are specific to Japan.