WASHINGTON — In 2006, three years after the Russian government had charged Mikhail Khodorkovsky — then the country's wealthiest businessman — with fraud and moved to break up his Yukos oil company, U.S. diplomats had had enough.

Gazprom, which grew out of the former Soviet Union's state gas ministry, had been busy buying up Yukos' far-flung empire, stoking American fears that soon Russia and its tough leader, Vladimir Putin, would control virtually all of the natural gas flowing to Europe.

The United States wanted to stop that from happening. So the American embassy in Slovakia hired a Texas-based oil consultant and began secretly advising the Slovakian government on how to buy the 49 percent stake Yukos had held in Transpetrol, the Slovakian oil pipeline company.