Volkswagen's decision to equip 11 million vehicles with devices to cheat emissions tests worldwide has sent the company into an acute corporate crisis in recent days. But on Dec. 2, 2014, the company assured U.S. and California regulators that its engineers had a straightforward solution.

Volkswagen told officials then that a software change would remedy the overflow of pollution emitted by its diesel cars, according to state and federal letters to the company. At the time, Volkswagen proposed a “voluntary recall” of about 500,000 vehicles. State and federal officials approved the plan.

That fix was either a technical failure or, some officials said, another ruse.

By May of this year, California tests showed that “the recall calibration did reduce emissions to some degree but NOx [nitrogen oxide] emissions were still significantly higher than expected.”

The extent of the recall, which is discussed in letters between government and company officials, is unknown. Company officials wouldn’t say this week how many recall notification letters Volkswagen issued to consumers, and how many consumers brought their cars in for the purported fix.

But the incident is one of the highlights in what federal and state officials have likened to a cat-and-mouse game between regulators and one of  the world’s largest automakers.

“They basically ran out of excuses,” said Stanley Young, spokesperson for the California Air Resources Board. “They would say the tests weren’t at the right temperature, or some other issue. We had them in [to our offices] several times.”

Volkswagen admitted to installing software designed to cheat emissions tests in about 11 million cars worldwide. Here's what you need to know about the scandal. (Jorge Ribas/The Washington Post)

Environmental Protection Agency administrator Gina McCarthy said on Tuesday that the agency would step up efforts to look for cheating by other manufacturers, though she suggested that Volkswagen was an “outlier” in the extent of its cheating to defeat emissions standards.

“We are not going to sit around and worry about whether or not there are other devices. We’re going to find it,” she said. “So right now we’re upping our game in terms of going out and doing the work to take a look at what we need to do with other vehicles.”

The company’s dire diesel troubles have erupted over just a few days, but they were a long time in gestation.

The regulations that shaped the design of the diesel engines were put in place more than 15 years ago, when the EPA finalized rules that raised the emission standards for diesel cars.

After a years-long grace period, the tough standards were phased in between 2004 and 2007. At the time, the government acknowledged that meeting the standards for NOx, or nitrogen oxide, as well as soot, would be difficult.

“Manufacturers have expressed concerns that diesel-fueled vehicles would have difficulty meeting NOx and particulate matter levels like those contained in today’s rule,” EPA officials wrote in issuing the new standards. “Clearly, these standards will be challenging.”

For a time, Volkswagen and other automakers stepped back from the U.S. diesel car market. It was a time to retool. Experts said the challenge of making a diesel engine clean enough for the U.S. standards -- without compromising how the engine works -- is very difficult.

“NOx is more of a challenge for diesel engines than it is for gasoline engines,” said John Storey, distinguished research and development staff member at Oak Ridge National Laboratory. “The strategies companies use to clean them up all have an impact on performance, fuel economy and maintenance.”

The engineering hurdles in controlling diesel emissions are high enough, some experts said, that they may provide an incentive for companies to skirt the rules.

“I can’t say I’m surprised,” John M. DeCicco, a research professor at the University of Michigan Energy Institute, said of the recent revelations. “The temptation to game the system with a defeat device is definitely high because of the technical challenges.”

In 2008, Volkswagen came back with a diesel version of the Jetta. The company touted it as an example of the “clean diesel.” It won raves, including “Green Car of the Year” at the Los Angeles Auto Show.

“After a three-year break that seemed interminable to fans, Volkswagen's back in the U.S. diesel-car market with a clean-burning diesel in its popular Jetta compact sedan,” USA Today told readers.

But what seemed like a breakthrough seems now, in light of the recent revelations, like a massive cheat.

The company’s two-liter diesel engines in the United States had been equipped with a “defeat device” that allowed the cars to pass federal emissions tests despite emitting more than 10 times the permitted amounts of NOx when it is on the road. Those emissions help produce smog on hot summer days, triggering a variety of health problems, particularly for asthmatics and other people with breathing difficulties.

As has been widely reported, the emission problems were detected in a May 2014 report by the International Council on Clean Transportation and West Virginia University.

The results were presented to the company and, several months later, in December 2014, the company told California regulators that they had a fix. They would “recalibrate” the software that runs the engine.

“Based on this meeting, VW initiated a voluntary recall. … This recall was claimed to have fixed among other things, the increased real world driving NOx issue,” according to a recent letter from the California Air Resources Board to the company.

By May of this year, California officials had run road tests on cars altered  under the recall. Despite the fix, the car showed higher than expected NOx emissions. California engineers also created a special dynamometer test, apparently to address company concerns that the previous testing was unfair.

California Air Resources Board “has determined that the previous recall did not address the high on-road NOx emissions, and also resulted in the vehicle failing certification standards,” it told the company this month.

McCarthy, speaking to reporters in Washington, acknowledged that the Volkswagen software ruse was “particularly difficult for us to detect. We haven’t found similar types before but we’ll take a look and make sure we’re attacking it successfully,” she said.

McCarthy defended the EPA’s regulatory standards as reasonable and said most other car manufacturers appear to have found a way to comply.

“This is about a NOx emission standard that was put in place a few decades ago. The industry knew what was happening. We feel pretty confident the industry has been innovating to be able to meet this standard,” she said. “So we don’t think that this problem is with the standards at all.”

McCarthy said she was gratified by Volkswagen’s aggressive response in acknowledging its mistakes, even as she criticized the company for its years of deliberate deception.

"I think it will be very difficult for Volkswagen to be looked at as anything other than a real outlier,” she said.

The Volkswagen scandal is spreading from the United States to South Korea and Europe. (Reuters)