Volkswagen management was warned as far back as May 2014 US officials might examine engine control software, according to a news agency report.

According to Reuters, citing two unnamed sources, a high-ranking worker told "senior" VW managers the scrutiny might come during an investigation into pollution levels.

The sources said a warning letter was sent to management over a year before the carmaker admitted publicly its cars used software to manipulate emission test results. Reuters noted that raised questions about how much senior managers knew about the scandal.

The US Justice Department is suing VWfor up to US$46bn for alleged violations of environmental law while regulators and prosecutors are looking to establish what role, if any, had been played by senior managers, including former chief executive Martin Winterkorn.

Volkswagen has admitted to cheating pollution tests but maintained only a small number of employees were to blame and that there was no indication that board members were involved.

According to the news agency, German newspaper Bild am Sonntag said a VW employee known internally as "Winterkorn's fireman" had notified superiors about the probe.

Two sources told Reuters they knew of the VW letter but it was not certain that Winterkorn, who resigned shortly after the scandal surfaced, had seen it.

The letter was sent from the carmaker's product quality and safety department, one source said.

A VW spokesman told Reuters the company does not comment on ongoing investigations and would report on its internal inquiry in the second half of April.