Prosecutors have searched premises at Volkswagen's headquarters in Germany as part of their criminal inquiry into the firm's emissions scandal.

The raids were carried out in Wolfsburg and at a number of other locations.

Officials said they were targeting information relating to the creation of the company's diesel engine software - designed to cheat emissions-testing.

In a statement announcing the raids, the prosecutors said the aim of the searches was to "secure documents and data storage devices" that could identify those involved.

VW has suspended four members of staff over the affair, which is set to lead to 11 million VW, Audi, Seat and Skoda vehicles being recalled worldwide in a process that could last until late next year.

The company's new chief executive Matthias Mueller said earlier this week that VW, which has already set aside €6.5bn to cover the costs of the scandal, was reviewing all its investment plans.

It is believed that recalls and potential legal action by regulators and car owners will raise that figure by a significant margin.

The bill is also likely to be exacerbated by falling sales as customers are expected to shy away from the manufacturer.

"We have broken the trust of our customers"

Meanwhile, the boss of Volkswagen US has admitted he was told in early 2014 about a "possible emissions non-compliance" at the company.

Michael Horn, the CEO of Volkswagen Group of America, has also apologised for the German carmaker's actions, describing them as "deeply troubling".

The apology and details about what Mr Horn knew of the emissions cheating are revealed in testimony he will present later today to a US House Committee investigating the scandal.

"I did not think something like this was possible at the Volkswagen Group," he says in the written testimony.

"We have broken the trust of our customers, dealerships, and employees, as well as the public and regulators".

Mr Horn said he was made aware of the "possible emissions non-compliance" following the publication of a West Virginia University study in spring 2014.

He said: "I was informed that EPA regulations included various penalties for non-compliance with the emissions standards and that the agencies can conduct engineering tests which could include 'defeat device' testing or analysis".

"I was also informed that the company engineers would work with the agencies to resolve the issue".

"Later in 2014, I was informed that the technical teams had a specific plan for remedies to bring the vehicles into compliance and that they were engaged with specific agencies about the process".

Mr Horn said the company takes "full responsibility for our actions and we are working with all relevant authorities in a cooperative way".

He added that VW is "determined to make things right" in the wake of the scandal.