The demand for ethanol fuel in the United States was stimulated by the discovery in the late 1990s that methyl tertiary butyl ether (MTBE), an oxygenate additive in gasoline, was contaminating groundwater. Due to the risks of widespread and costly litigation, and because MTBE use in gasoline was banned in almost 20 states by 2006, the substitution of MTBE opened a new market for ethanol fuel. This demand shift for ethanol as an oxygenate additive took place at a time when oil prices were already significantly rising. This shift also contributed to an expansion in the use of gasohol E10 and to a sharp increase in the production and sale of E85 flex vehicles since 2002.