German public prosecutors have searched Volkswagen's Wolfsburg headquarters as part of their investigation into the emissions scandal.

The prosecutor's office said they were looking for data linked to the manipulation of diesel emissions.

The search comes shortly before Michael Horn, VW's US boss, is due to appear before a US Congressional committee.

On Wednesday, he admitted he was aware early last year of the VW group's emissions cheating.

Michael Horn said he was told about a ”possible emissions non-compliance" in the spring of 2014.

German public prosecutors have searched Volkswagen's Wolfsburg headquarters as part of their investigation into the emissions scandal.

The prosecutor's office said they were looking for data linked to the manipulation of diesel emissions.

The search comes shortly before Michael Horn, VW's US boss, is due to appear before a US Congressional committee.

On Wednesday, he admitted he was aware early last year of the VW group's emissions cheating. Michael Horn said he was told about a "possible emissions non-compliance" in the spring of 2014.

The revelation was made in testimony to be presented to a committee of the US House of Representatives investigating the scandal on Thursday.

German prosecutors launched their investigation into the scandal last week after receiving about a dozen criminal complaints from citizens and one from VW itself.

They say they are trying to find out who was responsible for the alleged manipulation and how it was carried out.

Defeat device

Mr Horn said he was told about the emissions cheating after the publication of a study by West Virginia University.

"I was informed that EPA [Environmental Protection Agency] 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," he said.

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

Mr Horn said in the written evidence it was not until 3 September this year that Volkswagen told US authorities about the "defeat device" in emissions software in diesel vehicles for the model years 2009 to 2015.

'Deeply troubling'

The software allowed a vehicle to recognise whether it was being driven on the road or running in a test laboratory, and turn engine emissions controls on or off.

Mr Horn said the events had been "deeply troubling", adding: "I did not think that something like this was possible at the Volkswagen Group.

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

He said the company took full responsibility for its actions and was co-operating with all relevant authorities.

The executive will give evidence to the House Energy committee and Commerce subcommittee on oversight and investigations before being questioned by the politicians about the scandal, which affects half a million cars in the US.