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 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."
He was giving evidence before a US 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 co-operative way."
Mr Horn said he was told 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 to manipulate emissions tests at the beginning of September, just before the scandal was made public.
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.