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FRANKFURT, Germany (AP) — German automaker Daimler AG says prosecutors will search several of its offices in Germany as part of a preliminary investigation into suspected manipulation of diesel emission controls.

Daimler said Tuesday it was cooperating with the probe conducted in its home base of Stuttgart. The company had already disclosed the probe in its financial reports. It said the investigation focused on “known and unknown employees” of the company.

The company declined further comment.

Daimler said in its first-quarter earnings report that authorities in the U.S. and Europe had asked it for information about test results and the control systems used on its Mercedes-Benz cars. It said the U.S. Justice Department had asked the company to conduct an internal investigation.

Diesel emissions have come under closer scrutiny after Volkswagen AG admitted in September 2015 that it had equipped cars with software that detected when the vehicles were on test stands and turned up the emissions control, then turned controls off in everyday driving. Volkswagen has agreed to pay more than $16 billion in civil settlements and a $4.3 billion criminal penalty in the United States.

Fiat Chrysler has updated emission control software in 2017 models after U.S. environmental authorities said its vehicles emitted more pollution on the road than showed up in emissions tests.