German media reported Sunday that Volkswagen was warned years ago about the use of illegal tricks to defeat emissions tests.
The German automaker admitted last week that it used special software to fool U.S. emissions tests for its diesel vehicles. About 11 million VW diesel cars built since 2008 are affected by the scandal.
German weekly Bild am Sonntag reported that VW's internal investigation has found a 2007 letter from parts supplier Bosch warning Volkswagen not to use the software during regular operation.
Separately, the Frankfurter Allgemeine Sonntagszeitung reported that a Volkswagen technician raised concerns about illegal practices in connection with emissions levels in 2011. The weekly also cited VW's internal investigators.
A spokesman for Volkswagen declined to comment on the reports, saying that as a matter of principle the company wouldn't comment on what he called "rumors and speculation."
"Volkswagen is working with all its strength to conduct a thorough and merciless investigation of this matter," Andreas Lampersbach said in an email.
VW would hold those responsible for rigging the emissions test to account and discuss technical solutions for the problem with authorities, he said, citing a statement by the company's supervisory board. "Afterward we will provide a timetable to modify the vehicles of affected customers. This will take several weeks."
The world's biggest automaker is adding up the cost to its business and reputation of the biggest scandal in its 78-year history, having acknowledged installing software in diesel engines designed to hide their emissions of toxic gasses.
Countries around the world have launched their own investigations after the company was caught cheating on tests in the United States. Volkswagen says the software affected engines in 11 million cars, most of which were sold in Europe.
The company's internal investigation is likely to focus on how far up the chain of command were executives who were responsible for the cheating, and how long were they aware of it.
A spokesman for Bosch ROBG.UL said the company's dealings with VW were confidential.
German politicians have been adding to the pressure on Volkswagen, worried about the reputation of German industry.
"We need a guarantee that cars of German manufacturers are in line with the norms, without manipulation," Chancellor Angela Merkel's chief of staff Peter Altmaier told Der Tagesspiegel in an interview published on Sunday.
Environment Minister Barbara Hendricks said the scandal must not be allowed to tarnish "the made in Germany brand."
"If a global player from Germany violates environment protection rules that blatantly, this casts a shadow on the environment pledges of German companies,” she told Handelsblatt newspaper in an interview to be published on Monday.
Volkswagen and other European manufacturers have promoted "clean diesel" technology, benefiting from diesel's fuel economy but meeting stringent tests for emissions of toxins. But the suggestion that this was achieved by cheating on tests could affect the viability of the entire diesel sector and the fate of companies that have bet on it.