Volkswagen admitted to installing software designed to cheat emissions tests in about 11 million cars worldwide. While the cars passed emissions tests, they released nitrogen oxide up to 40 times the level allowed by federal law. Here's what you need to know about the scandal. (Jorge Ribas/The Washington Post)

Federal regulators are launching more aggressive testing of diesel-engine cars sold in the United States in the wake of the Volkswagen emissions-cheating scandal, the Environmental Protection Agency said Friday.

The EPA sent automakers a letter announcing expanded efforts to uncover “defeat devices” and other mechanisms to thwart air pollution laws, agency officials said at a news conference. But the officials said details of the new procedures will be kept confidential to make it harder for the industry to use technology to circumvent them.

The tests will include the monitoring of vehicles borrowed from individual consumers as well as extensive highway testing with equipment that can track a car’s emissions as it is being driven, the EPA said.

“We must continue to improve and adapt our oversight, and we will,” said Janet McCabe, acting assistant administrator for the Office of Air and Radiation. The additional monitoring will help to ensure that “the industry is competing on a level playing field and that consumers are getting what they paid for,” she said.

Regulators will be especially vigilant for technical tricks, such as the altered software that VW engineers used to fool the EPA’s pollution tests on nearly half a million cars, said Christopher Grundler, director of the EPA’s Office of Transportation and Air Quality.

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“We are not going to tell them what these tests are; they do not need to know,” Grundler said of automakers. “They only need to know that we will be keeping their vehicles longer and driving them more.”

The VW cheating scandal was initially discovered by outside contractors using portable monitors that measure emissions while a car is on the highway. Grundler said the EPA had been using the same devices for years but had focused its testing on large trucks, which account for far more air pollution than the relatively small number of diesel-fueled passenger cars in the United States.

But highway testing will become routine for all diesels, he said. And Canada’s environmental agency is joining the effort, he added.

“The testing is starting now,” Grundler said.

Agency officials repeated assurances that VW and Audi models with the “defeat devices” were safe to drive and said the manufacturer will eventually be required to make repairs at no cost to consumers, though it may be months before potential recall notices are issued.

VW has acknowledged that 11­ million of its vehicles worldwide, including nearly 500,000 in the United States, were involved in a years-long deception that allowed cars to pass emissions tests while releasing nitrogen oxide at up to 40 times the U.S. standard. Its chief executive, Martin Winterkorn, stepped down this week, and the company is under criminal investigation by the Justice Department and faces billions of dollars in fines.

VW, which has seen its stock plummet since the scandal broke, has set aside more than $7 billion to remedy the problems with its cars. It has pledged also to repair its reputation with customers.

Its board on Friday named Matthias Müller, chairman of Porsche AG, as the company’s new CEO. A statement said Müller will remain chairman of Porsche “until a successor has been found.”

Neither the EPA nor Volkswagen have said how long it will take to fix the cars in the United States with defeat devices. But it expects that 2015 model year cars will be patched “relatively quickly,” Grundler said.

Older versions of the Passat should be fixed soon after that, but other models, including Golfs, Jettas and Beetles, will take longer, Grundler said, because designing a solution “will require additional engineering development that will take longer.”

Moore is a freelance writer.