Six high-level Volkswagen employees from Germany were indicted in the US on Wednesday in the VW emissions-cheating scandal, while the automaker itself agreed to plead guilty to criminal charges and pay $4.3 billion - by far the biggest fine ever levied by the government against an automaker.

In announcing the charges and the plea bargain, Justice Department prosecutors detailed a large and elaborate scheme inside VW to commit fraud and then cover it up, with at least 40 employees allegedly involved in destroying evidence.

"Volkswagen obfuscated, they denied and they ultimately lied," Attorney General Loretta Lynch said.

VW installed software in diesel engines on nearly 600,000 VW, Porsche and Audi vehicles in the US that activated pollution controls during government tests and switched them off in real-world driving. The software allowed the cars to spew harmful nitrogen oxide at up to 40 times above the legal limit.

US regulators confronted VW about the software after university researchers discovered differences in testing and real-world emissions. Volkswagen at first denied the use of the so-called defeat device but finally admitted it in 2015.

Even after that admission, prosecutors said, company employees were busy deleting computer files and other evidence.

The fine easily eclipses the $1.2 billion penalty levied against Toyota in 2014 over unintended acceleration in its cars.

The German company pleaded guilty to conspiracy, obstruction of justice and importing vehicles by using false statements.

Under the agreement, VW must cooperate in the continuing investigation, which could lead to the arrest of more employees.

The automaker also agreed to the appointment of an independent monitor to oversee its compliance for three years.

Volkswagen previously reached a $15 billion civil settlement with environmental authorities and car owners in the US under which it agreed to repair or buy back up to a half-million of the affected vehicles.

The six supervisors indicted by a federal grand jury in Detroit were accused of lying to environmental regulators or destroying computer files containing evidence.