Volkswagen's US boss has admitted he was told in spring last year of the emissions cheating affecting millions of the company's vehicles.
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.
The detail emerged in testimony he is due to give to a committee of the US House of Representatives investigating the scandal on Thursday.
The company did not tell the US authorities until 3 September 2015 about the emissions "defeat device", Mr Horn said in the evidence.
He said the company took full responsibility for its actions and was co-operating with all relevant authorities.