Mr. Bryan has not been charged with any wrongdoing. In an interview, he said that he “never made a dime off any of the people I knew from the Ukraine, deliberately, because I didn’t want to violate any of the ethics rules.”

Mr. Bryan, 59, has spent most of his adult life in public service, including 17 years in the Army, followed by five years in civilian roles at the Defense Department before joining the Energy Department about a decade ago. He was eventually named a senior adviser in the department’s international affairs division, which oversaw the Ukraine work.

During some of the period being scrutinized, Mr. Bryan was designated a “special government employee.” The job category is intended to permit the government to place experts on the federal payroll for a limited period while allowing them to continue earning money in the private sector. But it has regularly generated controversy over real or perceived conflicts of interest under both Mr. Trump and his predecessors, most notably the case of Huma Abedin when she served as a special government employee at the State Department under Hillary Clinton.

While operating under that designation at the Energy Department, Mr. Bryan was affiliated with a Virginia consulting firm, ValueBridge International, and he remained with the firm after he left the Energy Department in mid-2016.

The overlap between his Energy Department work and ValueBridge’s efforts, revealed in documents and interviews, underscores the lure of big money that is often dangled in front of Washington officials and consultants by powerful foreign interests looking to shape American politics and policy. It also highlights the difficulty in discerning the motivations of those interests.

The whistle-blower complaint against Mr. Bryan was filed by Robert Ivy, who worked with Mr. Bryan on the Ukraine project when both men were officials at the Energy Department.