The Environmental Protection Agency has asked the Justice Department to investigate charges that General Motors Corp. used rigged cars to pass EPA emission-control tests at a Buick plant in Flint, Mich., last December.

The move came after GM accused the EPA on Tuesday of exceeding its authority by conducting a criminal investigation. On that day, EPA investigators attempting to enter the Buick plant were met by GM attorneys armed with request for a restraining order that would deny entrance to the EPA, because the agency is not empowered to conduct a criminal probe.

The EPA backed off on that threat of legal action, and decided to turn the matter over to the U.S. Attorney in Detroit.

The controversy began last December when EPA conducted with very little notice - "probably less than 24 hours advance warning," according to EPA spokesman Marlin Fitzwater.

But shortly afterward, according to Fitzwater, EPA received an anonymous letter "suggesting that the cars chosen for assembly that day were equipped with emission components that may have been unrepresentative of normalproduction vehicles."

The letter also claimed that the world's largest auto maker put the normal emission components - which would not have passed the EpA tests because of poor performance - back into cars coming off the line after the audit.

EPA official Benjamin R. Jackson sent a letter to GM on Jan.16, informing the company of the allegations and asking for a response.

"We received no response," said Fitzwater, "so we sent them another letter on Feb. 9, asking again."

After the second letter, GM lawyers called the EPA to ask for an extension. On Feb.28, the EPA wrote to GM, giving the company until March 10 to respond.