VW3 | EPA, Notice of Violation, September 18, 2015. Original scanned six-page notice: https://www.epa.gov/sites/production/files/2015-10/documents/vw-nov-caa-09-18-15.pdf
VW4 | EPA, "EPA, California Notify Volkswagen of Clean Air Act Violations," September 18, 2015. Original: https://www.epa.gov/archive/epa/newsreleases/epa-california-notify-volkswagen-clean-air-act-violations-carmaker-allegedly-used.html

[VW4, September 18, 2015 EPA announcement, allegations and investigative findings as attributed to EPA]
Today, EPA is issuing a notice of violation (NOV) of the Clean Air Act (CAA) to Volkswagen AG, Audi AG, and Volkswagen Group of America, Inc. (collectively referred to as Volkswagen). The NOV alleges that four-cylinder Volkswagen and Audi diesel cars from model years 2009-2015 include software that circumvents EPA emissions standards for certain air pollutants.

As described in the NOV, a sophisticated software algorithm on certain Volkswagen vehicles detects when the car is undergoing official emissions testing, and turns full emissions controls on only during the test. The effectiveness of these vehicles' pollution emissions control devices is greatly reduced during all normal driving situations. This results in cars that meet emissions standards in the laboratory or testing station, but during normal operation, emit nitrogen oxides, or NOx, at up to 40 times the standard. The software produced by Volkswagen is a "defeat device," as defined by the Clean Air Act.

EPA and CARB uncovered the defeat device software after independent analysis by researchers at West Virginia University, working with the International Council on Clean Transportation, a non-governmental organization, raised questions about emissions levels, and the agencies began further investigations into the issue. In September, after EPA and CARB demanded an explanation for the identified emission problems, Volkswagen admitted that the cars contained defeat devices.

[VW5 | Volkswagen AG Rule 11 Plea Agreement, entered January 2017; exhibit 2 is VW's stipulated statement of facts. Original: https://www.justice.gov/opa/press-release/file/924436/dl]
[VW5, Agreement section 1.E, original PDF p. 7]
The Defendant is pleading guilty because it is guilty of the charges contained in the Third Superseding Information. The Defendant admits, agrees, and stipulates that the factual allegations set forth in Exhibit 2 (the Statement of Facts) are true and correct, that it is responsible under the laws of the United States for the acts of its employees described in Exhibit 2, and that the facts set forth in Exhibit 2 accurately reflect the Defendant's criminal conduct and provide a factual basis for the guilty plea.

[VW5, Exhibit 2 paragraphs 52-53, PDF p. 63]
52. In or around March 2014, certain VW employees learned of the results of a study undertaken by West Virginia University's Center for Alternative Fuels, Engines and Emissions and commissioned by the International Council on Clean Transportation (the "ICCT study"). The ICCT study identified substantial discrepancies in the NOx emissions from certain 2.0 Liter Subject Vehicles when tested on the road compared to when these vehicles were undergoing EPA and CARB standard drive cycle tests on a dynamometer. The results of the study showed that two of the three vehicles tested on the road, both 2.0 Liter Subject Vehicles, emitted NOx at values of up to approximately 40 times the permissible limit applicable during testing in the United States.

53. Following the ICCT study, CARB, in coordination with the EPA, attempted to work with VW to determine the cause for the higher NOx emissions in the 2.0 Liter Subject Vehicles when being driven on the road as opposed to on the dynamometer undergoing standard emissions test cycles. To do this, CARB, in coordination with the EPA, repeatedly asked VW questions that became increasingly more specific and detailed, as well as conducted additional testing themselves.

[VW5, Exhibit 2 paragraph 55, PDF p. 64]
55. Throughout 2014 and the first half of 2015, Supervisors A, D, and E, and others, continued to offer, and/or cause to be offered, software and hardware "fixes" and explanations to U.S. regulators for the 2.0 Liter Subject Vehicles' higher NOx measurements on the road without revealing the underlying reason-the existence of software designed to detect, evade and defeat U.S. emissions tests.

Original EPA notice is a scanned document: the contemporaneous EPA press release VW4 supplies searchable quotations. The agreement records a corporate admission; it does not retroactively make the specific defeat-device mechanism knowable to a May 2014 reader.

VW6 | ICCT, \"EPA’s notice of violation of the Clean Air Act to Volkswagen,\" September 18, 2015. Original: https://theicct.org/epas-notice-of-violation-of-the-clean-air-act-to-volkswagen-press-statement/

[VW6, September 18, 2015, Fact Sheet: Light-duty diesel in-use tests]
In 2013, the ICCT contracted with the Center for Alternative Fuels, Engines and Emissions at West Virginia University to conduct in-use emissions tests on three light-duty diesel vehicles: a Volkswagen Jetta, a VW Passat, and a BMW X5.

The three test vehicles were certified to US-EPA Tier2-Bin5 and California LEV-II ULEV emissions limits and were equipped with NOx after-treatment technologies: the Jetta was tested with a lean-NOx trap (LNT), the Passat with a urea-based selective catalytic reduction (SCR) system, and the BMW X5 with an SCR system.