Volkswagen is thought to have made several versions of its "defeat device" software to rig diesel emissions tests, implying a complex deception by the German carmaker, according to people familiar with the matter.
Speaking to Reuters, the sources - who include a VW manager and a US official close to the investigation - said that over a period of seven years Volkswagen altered its illegal software for four engine types. Spokespersons for VW in Europe and the United States declined to comment, citing ongoing investigations by the company and authorities in both regions.
Asked about the number of people who might have known about the cheating, a spokesman at company headquarters in Wolfsburg, Germany, said: "We are working intensely to investigate who knew what and when, but it's far too early to tell."
Automotive industry experts have said that having several versions of the defeat device makes it more likely that a range of employees were involved. For example, software technicians would have needed regular funding and knowledge of engine programmes, they said.
The number of people involved is a key issue for investors because it could affect the size of potential fines and the extent of management change at the company.
VW, Europe's biggest carmaker, has been criticised by some lawmakers and analysts for blaming a small number of individuals for the banned software installed in up to 11 million vehicles worldwide, even while investigations continue.
Its US chief, Michael Horn, told lawmakers earlier this month that he believed "a couple of software engineers" were responsible, while a letter dated October 8 from VW to the European Parliament blamed "the misconduct of a few people."
VW admitted publicly on September 18 that it had used software that could tell when a diesel vehicle was being tested and temporarily lower its toxic emissions to pass US regulations. The scandal has wiped around a quarter off its stock market value and forced out its long-time chief executive.
When it started using defeat device software in 2008, VW installed it with the EA189 diesel engine. The software was subsequently added to the newer EA288 engine.
"VW would have had to reconfigure the software for each generation of engines," said the US official close to the investigation.
In older diesel models, VW used so-called Lean NOx Traps designed to reduce toxic nitrogen oxides in engine exhaust. From around 2012, it introduced a more sophisticated and expensive system called Selective Catalytic Reduction.
Meanwhile, police in France have carried out searches of Volkswagen offices in France, as part of the widening investigation. Volkswagen's main French office in Villers-Cotterets, northeast of Paris, and another VW office at Roissy, near Paris' main airport, were searched and computer material seized, an official at the Prosecutor's office said. The Paris Prosecutor is conducting a preliminary inquiry into suspected "aggravated deception" by Volkswagen.
A Volkswagen spokesman confirmed the company had been searched by French police on Friday and was cooperating with the authorities, declining to comment further.
Volkswagen has said up to 11 million vehicles worldwide could contain banned software and on Thursday announced it would recall around 8.5 million vehicles in the European Union.
Despite the diesel emissions scandal, an independent online survey of 1,000 German citizens by market research firm Prophet showed that two-thirds of those surveyed still believe Volkswagen builds "outstanding" cars.
Sixty-five percent said they either fully or largely agreed the scandal was overdone and that VW still made excellent cars. Six out of 10 said they did not believe the "Made in Germany" label would be damaged by the scandal in the long term, while 63 percent believed the affair would soon be forgotten.