Volkswagen initially denied it was trying to game the inspections, attributing the higher emissions readings to "various technical issues and unexpected in-use conditions," the EPA said in its formal notice of violations on Friday.
The stonewalling continued until the agency threatened to withhold certification for the carmaker's 2016 models, the EPA said.
"Only then did VW admit it had designed and installed a defeat device" that purposely lowered emissions while it was being inspected, the agency said.
Volkswagen Chief Executive Martin Winterkorn . . . said on Sunday he was "deeply sorry" for the breach of U.S. rules and ordered an investigation