JUDY WOODRUFF: And now the scandal enveloping Volkswagen.

Today, the company said it would recall 11 million diesel cars worldwide, after admitting they had been rigged to cheat emissions tests. The company's stock price continues to plummet. Last week, its CEO resigned. And this week, German prosecutors said they were considering criminal charges against him.

William Brangham has more on the unlikely way it all began.

WILLIAM BRANGHAM: We take a closer look at how Volkswagen got caught, and what it all means, with one of the engineers who helped catch the automaker.

John German is a senior fellow at the International Council on Clean Transportation, a nonprofit dedicated to improving vehicle emissions and one that provides research to regulators all over the world.

So, John German, I wonder if you would just start off by telling me, how did this revelation come about?

JOHN GERMAN, International Council on Clean Transportation: Well, we were actually just going routine testing.

This is an outgrowth of our work in Europe. And it's been known in Europe for five to 10 years that diesel cars have high in-use NOx emissions.

ICCT has been working with some other groups to — on this. We have done some testing there. And our director of our European office, Peter Mock, had the bright idea that we should test some vehicles in the U.S. And our thought was, because the U.S. has the most stringent emissions standards in the world, and because EPA and CARB have a lot of legal authority and experience and do effective enforcement, that the diesel cars in the U.S. would be clean, and that we would take this data back to Europe and say, hey, they can do it in the U.S., how come you can't do it in Europe?

So, we were as surprised as anybody when we got the results.

WILLIAM BRANGHAM: So, Volkswagen installed this software that turned on the emissions system when the computer was being tested — when the car was being tested, and then shut it off when the car was out on the road and a regular driver was driving it.

Now, I understand there is a great deal of complexity in the computer systems in our cars today, but how does it know? How did the software know that it was being tested?

JOHN GERMAN: There's no way for us to know exactly what V.W. did, but there's a lot of potential ways that a computer can recognize that it's on a test.

The tests are run in a laboratory, and the vehicle is strapped down and run on some rollers. So, it's stationary. The rear wheels don't turn. Well, if it's a front-wheel drive car, the non-drive wheels don't turn. The steering rack never turns, because the vehicle doesn't turn. It's bolted down.

The test is always run at the same temperature. The test is always run when the engine is cold at the start. And another way is that the test, it always follows a prescribed drive cycle. So, in other words, the speed, every second is known. And the computer can look for that, too.

WILLIAM BRANGHAM: Is there a concern in your mind that this could be a bigger issue than just Volkswagen?

JOHN GERMAN: Yes.

The — there is — we have no data, we have no information that suggests that any other manufacturer has been doing the same thing, but it's the right question to ask, especially since the U.S. has the best regulators and the most stringent standards in the world.

So, we definitely think that government agencies worldwide need to investigate to see if other manufacturers are doing the same thing.

WILLIAM BRANGHAM: In your mind, is this a sense of the fox guarding the henhouse? I mean, I understand that the complexity of these systems seem to me to allow the carmakers to get away with more and more nefarious activity. I mean, is that a concern of yours?

JOHN GERMAN: It would be in any country that doesn't have good legal authority and enforcement.

As you say, V.W. got away with this for a while in the U.S., but look at the repercussions once they got caught. And so it's just — the sheer magnitude of the fines and penalties and public opinion that they could face, that is the real deterrent. The agencies just have to — worldwide just have to do enough to make sure that there is some chance that they could get caught.

WILLIAM BRANGHAM: I mean, are you confident at all that this, the blowback from this scandal will deter other automakers from trying to cheat like this?

JOHN GERMAN: Absolutely.

And you already have other manufacturers scrambling to make sure that they're not accused of the same thing.

WILLIAM BRANGHAM: John German, the International Council on Clean Transportation, thank you very much for joining us.

JOHN GERMAN: Right. You're welcome. Thank you.