Connecticut Attorney-General George Jepsen has blasted Volkswagen for citing German law to withhold documents from a group of US state attorneys-general investigating the carmaker's use of illegal diesel emissions software.
"I find it frustrating that, despite public statements professing cooperation and an expressed desire to resolve the various investigations that it faces following its calculated deception, Volkswagen is, in fact, resisting cooperation by citing German law," Jepsen said in a statement on Friday.
A source briefed on the matter said the world's second-largest carmaker was withholding emails between its executives and other communications regarding the emissions scandal.
Volkswagen is citing German privacy law in refusing to turn over emails and other communications between its executives, the source said.
The emails were sought by a group of 48 US state attorneys-general investigating excess emissions in 580,000 US diesel cars.
The carmaker declined to say if it is withholding documents.
"We are in permanent exchange with US authorities and are cooperating closely with them. We are not commenting on ongoing investigations," a spokesman at Wolfsburg-based VW said.
Jepsen said state A-Gs will work "to hold Volkswagen accountable for its behaviour to the extent possible under the law, and we will seek to use any means available to us to conduct a thorough investigation of Volkswagen's conduct".
The US Justice Department and German prosecutors are also investigating VW's conduct.
On Monday, the Justice Department filed a civil suit against VW seeking up to $US46 billion ($A65.75 billion) for violating the Clean Air Act in allowing excess emissions in 580,000 diesel vehicles.
A criminal investigation is still ongoing.
The Justice Department's suit said the government's "efforts to learn the truth about the (excess) emissions ... were impeded and obstructed by material omissions and misleading information provided by VW entities".
VW has blamed the deception on a small group of employees.
