From The cigarette controversy by Cummings KM, Brown A, and O'Connor R. Cancer Epidemiol Biomarkers Prev. 2007 Jun 16(6):1070-6.

The tobacco companies knew and for most part accepted the evidence that cigarette smoking was a cause of cancer by the late 1950s. In 1994, heads of the major U.S. tobacco companies testified before Congress that the evidence that cigarette smoking caused diseases such as cancer and heart disease was inconclusive, that cigarettes were not addictive, and that they did not market to children. Less than 1 month after this testimony, a box containing confidential documents from the Brown & Williamson Tobacco Corporation was delivered to the University of California at San Francisco. What was revealed in these documents was evidence that the tobacco industry had for decades known and accepted the fact that cigarettes caused premature death, considered tobacco to be addictive, and that their programs to support scientific research on smoking and health had been a sham
The documents also reveal that the tobacco companies helped manufacture the smoking controversy by funding scientific research that was intended to obfuscate and prolong the debate about smoking and health. Today, the tobacco companies acknowledge that smoking is a cause of disease, but they have not materially altered the way they do business.

DNA evidence vs epidemological evidence