Disgraced former Volkswagen boss Martin Winterkorn faces a grilling by German lawmakers Thursday, as accusations grow of top executives at the beleaguered auto giant colluding to cover up emissions cheating.

The former CEO's testimony will be "particularly important" in light of the latest allegations against the VW group, said Herbert Behrens, who heads a special parliamentary committee investigating the "dieselgate" scandal.

Winterkorn resigned in September 2015, days after the VW group admitted it had installed software in 11 million diesel vehicles worldwide to dupe emissions tests and make the cars seem less polluting than they were.

A fastidious perfectionist with the nickname "Mr Quality", Winterkorn had once boasted jokingly in an interview: "I know every screw in our cars."

But the former chief executive said he knew nothing of the pollution cheating scam.

US investigators have turned up the heat on Volkswagen in recent weeks, revealing that they believe VW top brass were aware of the cheating as far back as July 2015.

They also arrested VW executive Oliver Schmidt, formerly responsible for US compliance issues, and charged him with fraud and conspiracy over the dieselgate controversy.

"The arrest of a VW manager and his declarations to the FBI are directly relevant to the work of the parliamentary committee," Behrens said in a statement last week, adding that it was essential to establish "when the VW board was informed" of the cheating.

- 'Authorised concealment' -

According to the FBI, Schmidt and other Volkswagen employees in July 2015 briefed senior executives at its German headquarters of the defeat device, saying regulators were not aware of the mechanism.

"Rather than advocate for disclosure of the defeat device to US regulators, VW executive management authorised its continued concealment," the FBI said.

Volkswagen has agreed to plead guilty to conspiracy to defraud the United States as well as to obstruction of justice for destroying documents related to the scheme.

As part of the deal, it has also agreed to pay $4.3 billion (four billion euros) in civil and criminal fines.

But according to the final settlement running to 86 pages, the group does not clarify who holds responsibility within the company for the scandal.

The issue carries importance in Germany, where investigators have placed Winterkorn under investigation as they seek to establish the exact chain of responsibility in the scam.

The probe is looking at the allegation of fraud in the sales of vehicles with manipulated emission values.

In addition, prosecutors are examining if management had divulged existence of the scandal later than they were legally obliged to under stock market rules, thereby essentially manipulating stock prices.

More than 1,400 shareholders are also suing for damages worth a total 8 billion euros after the announcement of the scam wiped out some 40 percent of VW's market capitalisation in days.

- 'Something illegal' -

The group has not budged on its timeline of events -- saying that top management was informed about the scandal only at "the end of August, early September 2015".

But on Sunday, German daily Bild said that during a meeting organised by VW management on July 27, 2015 and attended by Winterkorn, a participant raised "the fact that something illegal had been installed in our vehicles".

The participant then asked what strategy the group should adopt vis-a-vis the information, reported the daily.

That appeared to square with the timeline given by the FBI.

Meanwhile, Sueddeutsche Zeitung and regional broadcasters WDR and NDR reported that two witnesses had told US authorities that they had raised the issue directly with Winterkorn as early as 2012 and 2014.

The parliamentary committee is also trying to determine whether government officials knew about the scandal before it became public knowledge.

Vice-Chancellor Sigmar Gabriel has already appeared before the panel, while Chancellor Angela Merkel is to be questioned on March 8.

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