Neocom Systems did not come up at Mr. Manafort’s first trial, nor was it mentioned in the plea agreement he signed on Friday. But Rick Gates, a former employee who became the star witness for the prosecution, testified that Ukrainian oligarchs had paid millions of dollars to Mr. Manafort through an array of offshore companies with obscure names like Novirex, Firemax and Telmar.

Mr. Kaseyev said he first became aware of his unwitting role in the creation of at least three Ukrainian front companies a decade ago, when the tax police contacted him in 2007 about his purported $30 million tax liability at Regional Insurance Union, a company he had never heard of. His passport had been stolen in a burglary a year earlier.

The scale of unpaid taxes, he said, suggested that there were larger sums moving through the company.

A slender, soft-spoken young man then just getting his start as a beautician, and hence unlikely to have amassed such wealth, Mr. Kaseyev said he was able to quickly convince the police of the identity theft. Yet the companies continued to be used in deals, and one, April Limited, still appears to be open for business, a corporate registry shows.

Some homeless men have achieved a measure of fame among activists who track corruption in the former Soviet states because they pop up so often at the head of multimillion dollar companies.

One man identified in the Ukrainian media as a homeless Latvian named Erik Vanagels has been listed as the owner of hundreds of companies in Britain, Cyprus, Ireland, New Zealand, Panama and elsewhere. Companies in the network also helped finance the private zoo and sprawling estate of the former Ukrainian president, Viktor F. Yanukovych, who was the main client of Mr. Manafort.