The request from the Environmental Protection Agency (EPA)  follows up on a civil class-action lawsuit filed earlier this month that said 14 Mercedes diesel models contain technology that spew excess emissions of nitrogen oxide, while concealing the mechanism from regulators.
The suit from plaintiffs firm Hagens Berman takes aim at Mercedes's BlueTEC diesel technology, which the luxury automaker has touted for its environmental benefits.
"We know about the lawsuit," said EPA spokeswoman Julia Valentine. "We have contacted Mercedes and requested the test results for the US diesel engines."
A Mercedes spokeswoman said the carmaker, owned by Daimler, had received the EPA request and is preparing a response.
"We consider this class-action suit to be unfounded," she said. "We will defend ourselves by all legal means." "We take the protection of the environment very seriously and appreciate the past trust and cooperation with US regulatory agencies."