On a leafy side street off Independence Square in Kiev is an office used for years by Donald J. Trump’s campaign chairman, Paul Manafort, when he consulted for Ukraine’s ruling political party. His furniture and personal items were still there as recently as May.

And Mr. Manafort’s presence remains elsewhere here in the capital, where government investigators examining secret records have found his name, as well as companies he sought business with, as they try to untangle a corrupt network they say was used to loot Ukrainian assets and influence elections during the administration of Mr. Manafort’s main client, former President Viktor F. Yanukovych.

Handwritten ledgers show $12.7 million in undisclosed cash payments designated for Mr. Manafort from Mr. Yanukovych’s pro-Russian political party from 2007 to 2012, according to Ukraine’s newly formed National Anti-Corruption Bureau. Investigators assert that the disbursements were part of an illegal off-the-books system whose recipients also included election officials.

The developments in Ukraine underscore the risky nature of the international consulting that has been a staple of Mr. Manafort’s business since the 1980s, when he went to work for the Philippine dictator Ferdinand Marcos. Before joining Mr. Trump’s campaign this spring, Mr. Manafort’s most prominent recent client was Mr. Yanukovych, who — like Mr. Marcos — was deposed in a popular uprising.

Before he fled to Russia two years ago, Mr. Yanukovych and his Party of Regions relied heavily on the advice of Mr. Manafort and his firm, who helped them win several elections. During that period, Mr. Manafort never registered as a foreign agent with the United States Justice Department — as required of those seeking to influence American policy on behalf of foreign clients — although one of his subcontractors did.

It is unclear if Mr. Manafort’s activities necessitated registering. If they were limited to advising the Party of Regions in Ukraine, he probably would not have had to. But he also worked to burnish his client’s image in the West and helped Mr. Yanukovych’s administration draft a report defending its prosecution of his chief rival, Yulia V. Tymoshenko, in 2012.