A criminal investigation is being carried out into Vauxhall Zafira fires, a government agency has confirmed.
It is the first time the manufacturer has been investigated formally in relation to the fires.
The Driver and Vehicle Standards Agency (DVSA) wrote to the Commons Transport Select Committee stating that it is investigating Vauxhall in partnership with trading standards officers in Luton, where the firm's headquarters are based.
Campaigners claim more than 300 Zafiras have caught fire in recent years, often leaving their occupants with just a few seconds to escape.
Some of the fires were serious enough to destroy entire vehicles.
A damning report by the committee found the manufacturer was too slow to begin a full investigation into the fires and then acted prematurely in attributing the problem to improper repairs by third parties.
Concerns that there was a distinctive pattern of fires were first raised within Vauxhall in 2014, but it did not begin an investigation until August 2015.
The DVSA's investigation is attempting to reveal who at the company had knowledge about the problem and when they found out.
DVSA head of counter-fraud and investigations Andy Rice told the Press Association: "DVSA's priority is to protect you from unsafe drivers and vehicles.
"We have made it clear to manufacturers that to protect consumers, they should swiftly rectify problems and meet their obligations under the code of practice.
"DVSA will take the necessary action against any manufacturers who fail to comply with their obligations."
DVSA is working along trading standards officers at Luton Borough Council as it does not have the authority to launch its own criminal prosecutions.
The investigation was launched in October last year.
Vauxhall said the fires were caused by unauthorised repair of the thermal fuse in the people carriers' blower resistor, and in December 2015 it recalled 226,000 Zafira B vehicles for an inspection.
On February 2 2016 the manufacturer became aware of fires in vehicles after they had been recalled, but it was not until the third week of that month that it informed the DVSA, according to the select committee. A second recall was announced in May 2016.
A spokesman for Vauxhall said 92% of affected vehicles have been recalled, which is "virtually everything on the road".
Steve Gooding, director of motoring research charity the RAC Foundation, said: "Road safety is as much a matter for auto companies as for the people who drive their products.
"But while apportioning blame is important when things go wrong, the first priority must always be limiting harm through the swift identification of faults followed by their rapid rectification."