Media playback is unsupported on your device Media caption VW boss Michael Horn: "I would like to offer a sincere apology"

Volkswagen's US boss has made a "sincere apology" for installing "defeat devices" to cheat emissions tests on its diesel cars.

Michael Horn, chief executive of VW's US operation, said the events were "deeply troubling".

However, he said the decision to use the devices was not one made by the company's board, but by individuals.

Mr Horn said: "My understanding is that it was a couple of software engineers who put these in."

He said that three people had been suspended following the scandal, but they could not be named for legal reasons.

The executive was giving evidence before the House of Representatives Energy and Commerce committee.

Sceptical

Many of the committee members were sceptical. New York Congressman Chris Collins said: "VW is trying to get us to believe this is the work of a couple of rogue engineers. I don't believe it."

At one point, Mr Horn himself said he found it hard to believe. Texas Congressman Joe Barton asked him: "Do you really believe, as good, as well-run as Volkswagen has always been reputed to be, that senior level corporate managers, administrators, had no knowledge for years and years?"

Image copyright Getty Images Image caption Volkswagen US chief Michael Horn at the New York motor show in April

Mr Horn replied: "I agree it's very hard to believe. And personally I struggle [to believe it] as well."

Several of the committee members admitted they had owned VW cars and held a deep affection for them, which prompted some angry questioning.

Vermont Congressman Peter Welch asked Mr Horn: "How do you call yourself a member of the human race when you poison the human race?"

'Full responsibility'

Mr Horn 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.

"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 co-operative way."

Mr Horn told the panel he was informed about a "possible emissions non-compliance" in the spring of 2014.

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

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.

VW will offer a financial package to American dealers to help them through the crisis, but expects it will take "one to two years minimum" to fix the US cars.

Later on Thursday the state of Texas said it was taking legal action against Volkswagen over the marketing of its supposedly clean diesel vehicles, alleging that Volkswagen violated a state law prohibiting deceptive trade practices.

Image copyright AFP

Prosecutors

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.

Mr 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.

But he told the members of the committee: "I had no idea what a defeat device was or that Volkswagen used them."

Volkswagen told US authorities on 3 September this year about the "defeat device" in emissions software in diesel vehicles for the model years 2009 to 2015.