CHICAGO – More than a decade ago, the U.S. Environmental Protection Agency helped develop a technology that ultimately was used by an independent laboratory to catch Volkswagen’s elaborate cheating on car emissions tests. But EPA used the technology primarily to test trucks rather than passenger cars because such heavy equipment was a much bigger polluter.

That decision meant that the U.S. regulator missed its best chance to foil the German carmaker’s deception early on. The portable emissions measurement systems that EPA pioneered might have subjected VW diesel cars to on-road tests and discovered they were spewing up to 40 times the allowable levels of key pollutant nitrogen oxide under normal driving conditions.

Without that test, VW was virtually home free and evaded detection for seven years.

“If EPA had used the technology back then (on diesel cars), we could have caught it,” said Margo Oge, who was director of the EPA’s office of Transportation and Air Quality at the time and headed the office for 18 years until 2012. But she doesn’t regret EPA’s decision to focus that technology on manufacturers of trucks and heavy equipment, which had a record of cheating on tests and accounted for a much bigger portion of U.S. pollution than the nascent diesel car business.

Interviews with former and current EPA officials and other auto and environmental experts suggest that although the U.S. has the world’s toughest auto emissions standards, federal and state regulators don’t have the resources to conduct the kind of comprehensive tests that might have nabbed VW, and they rely on automakers to self-report data in a kind of honor system.

“They trust the auto companies to tell the truth. And the auto companies have proven time and again that they don’t tell the truth,” said Dan Becker, director of the Safe Climate Campaign and a veteran of the fight for tougher car emissions regulation. “We can’t allow the students to test themselves and submit their own grades.”