Volkswagen USA CEO Michael Horn has made a “sincere apology” for installing “defeat devices” to cheat emissions tests on the automaker’s diesel cars.

Michael Horn said the events were “deeply troubling”.

He added: “I did not think that something like this was possible at the Volkswagen group.

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

Michael Horn was giving evidence before a Congressional committee.

He said: “Let me be very clear: we at Volkswagen take full responsibility for our actions and we are working with all the relevant authorities in a cooperative way.”

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

However, he said he first learned about so called defeat devices being installed on VW diesel cars to manipulate emissions tests at the beginning of September, just before the scandal was made public.

Meanwhile, 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 defeat devices.

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.

Michael Horn said he was told about problems with VW’s diesel cars meeting US emissions tests 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.

He told the members of the committee: “I had no idea what a defeat device was or that Volkswagen used them.”

Michael Horn said in the written evidence it was not until September 3, 2015, that Volkswagen told US authorities about the “defeat device” in emissions software in diesel vehicles for the model years 2009 to 2015.

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

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

Michael Horn gave 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.