All this was possible because environmental regulators had tools and powers at their disposal that another area of enforcement — auto safety regulation — does not have, despite the efforts of lawmakers, consumer advocates and, more recently, auto safety regulators themselves. Often, the auto industry has beaten back more stringent laws.

The National Highway Traffic Safety Administration, for example, can impose a maximum penalty of $35 million on an automaker that flouts safety regulations — a relatively low sum for a company like General Motors, which last year paid such a fine for a defect that has now been linked to at least 124 deaths.

By contrast, under the Clean Air Act, Volkswagen, the world’s largest automaker, could be fined as much as $37,500 for each recalled vehicle, for a possible total penalty of as much as $18 billion.

“The Clean Air Act statutory scheme gives E.P.A. more power and flexibility to move more quickly than N.H.T.S.A.,” said Carl Tobias, a law professor at the University of Richmond, who has studied the government’s response to auto safety issues. “E.P.A. also seems more tough-minded and savvy about how to be effective in this arena.”

Still, the time it takes to investigate auto companies is often extensive, and getting the facts can be a challenge. “Even E.P.A. took a year to finally crack this case open,” he said.