For decades, oil and gas industry executives as well as regulators have maintained that a drilling technique known as hydraulic fracturing, or fracking, that is used for most natural gas wells has never contaminated underground drinking water.

The claim is based in part on a simple fact: fracking, in which water and toxic chemicals are injected at high pressure into the ground to break up rocks and release the gas trapped there, occurs thousands of feet below drinking-water aquifers. Because of that distance, the drilling chemicals pose no risk, industry officials have argued.

“There have been over a million wells hydraulically fractured in the history of the industry, and there is not one, not one, reported case of a freshwater aquifer having ever been contaminated from hydraulic fracturing. Not one,” Rex W. Tillerson, the chief executive of ExxonMobil, said last year at a Congressional hearing on drilling.

It is a refrain that not only drilling proponents, but also state and federal lawmakers, even past and present Environmental Protection Agency directors, have repeated often.