Volkswagen’s admission Friday that it built at least 482,000 diesels for the U.S. market with software designed to fake EPA emissions tests sent shock waves through the global auto industry, while sparking a personal apology from VW’s chief executive.

Yet the whole probe was kicked off by a small advocacy group with experience in testing whether diesel cars were meeting their legal obligations — and those tests suggest VW isn’t alone, something U.S. regulators will now pursue.

The charges by the EPA sparked an apology from VW CEO Martin Winterkorn on Saturday, saying “I personally am deeply sorry that we have broken the trust of our customers,” and vowing VW would order its own external probe while cooperating with authorities. That was before VW’s shares plunged more than 20 percent on Monday, after European authorities said they too would re-examine VW’s vehicles.

Winterkorn was the head of the VW brand at the time the vehicles with the emissions switches were built. Such engineering work would have been done in Europe, as VW’s U.S. arm focuses on marketing and sales.

The automaker has stopped selling what few 2015 model-year diesels it had on U.S. lots following the EPA announcement, and due to the dispute does not have approval to sell 2016 diesels, which have made up roughly a fifth of U.S. sales. VW has also stopped selling used 2-liter diesels until the company comes up with a fix that satisfies regulators.

All of which began with a few road trips in a VW Passat diesel up the West Coast, under the auspices of the International Council on Clean Transportation, a non-profit climate research group. Earlier this month, the ICCT released its study of 32 European diesel cars, measuring their emissions on both the driving conditions used by regulators and a tougher course that more closely mimics real-world conditions.

While the cars passed the regulated test cycle, almost all spewed more NOx than allowed in real-world driving, with three—one each from Volvo, Renault and Hyundai—belching far more than any other models. The data, the researchers say, pointed to a “serious compliance problem” with European diesels.

Story continues

Earlier data compiled by the ICCT found that the VW diesels had large emissions on certain tests. When the group’s U.S. associates tested the VW Jetta, Passat and diesel BMW X5, they found only the X5 kept its emissions within government standards in everyday driving. The Passat exceed the standards by 5 to 20 times, while the Jetta spewed 15 to 35 times more NOx than allowed.

BMW X5

The group told the EPA and the California Air Resources Board, which kicked off a year of wrangling with Volkswagen. VW did a recall of the affected vehicles in December with a software update, telling the agencies that it would bring them into compliance—except that vehicles were still emitting too much pollution.

Only on Sept. 3, after a warning about 2016 models being blocked, did VW confess the deception.

Why does this matter? After all, no one died, and in an era where “rolling coal” is a thing, letting Jettas smoke a little seems penny-ante. Firstly: diesel nitrogen oxides are the main fuel for smog and the health dangers it causes. Nitrogen oxides also happen to be the toughest pollutant to combat from an engine, requiring several expensive engineering fixes. And while diesel engines burn less fuel per mile than gasoline ones, running them smoothly often means creating even more NOx.

As the EPA and the California Air Resources Board have ratcheted down the NOx limits to cut smog, automakers and engine builders have fought back, and occasionally been caught bending the rules. In 1998, seven builders of heavy-duty diesel truck engines paid $1 billion to settle EPA charges after they built defeat devices into their products.

Today, most diesel passenger cars in the United States arrive fitted with an after-treatment system that requires occasional refills, known as diesel exhaust fluid or several brand names. These systems, including some installed on the VWs in question, have to be designed under EPA rules such that if they aren’t refilled properly, the engines won’t start.

After VW’s deception, the EPA, CARB and outside researchers will now be looking for other examples of cars that only meet their standards in the lab, but not on the road. The EPA official in charge of clean-air testing says the agencies will now review all light-duty diesels for a software switch similar to the one found in Volkswagens, according to Automotive News. And as the ICCT’s research shows, there’s more than just the suspects from Wolfsburg.