Michael Horn said he was told about a "possible emissions non-compliance" early 2014.
He was told of the problem after a study by West Virginia University was published.
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.
The company did not tell the US authorities until 3 September 2015 about the emissions "defeat device", Mr Horn said in the evidence.
I did not think that something like this was possible at the Volkswagen Group.
He said the company took full responsibility for its actions and was co-operating with all relevant authorities.