In July 2012, a shell company registered in the British Virgin Islands wired $20 million to an investment vehicle in the Cayman Islands that was controlled by a large American hedge fund firm.

The wire transfer was the culmination of months of work by a small army of handlers and enablers in the United States, Europe and the Caribbean. It was a stealth operation intended, at least in part, to mask the source of the funds: Roman Abramovich.

For two decades, the Russian oligarch has relied on this circuitous investment strategy — deploying a string of shell companies, routing money through a small Austrian bank and tapping the connections of leading Wall Street firms — to quietly place billions of dollars with prominent U.S. hedge funds and private equity firms, according to people with knowledge of the transactions.

The key was that every lawyer, corporate director, hedge fund manager and investment adviser involved in the process could honestly say he or she wasn’t working directly for Mr. Abramovich. In some cases, participants weren’t even aware of whose money they were helping to manage.

asked Congress for more resources as it helps to oversee the Biden administration’s sanctions program along with a new Justice Department kleptocracy task force. And on Capitol Hill, lawmakers are pushing a bill, known as the Enablers Act, that would require investment advisers to identify and more carefully vet their customers.

Mr. Abramovich has an estimated fortune of $13 billion, derived in large part from his well-timed purchase of an oil company owned by the Russian government that he sold back to the state at a massive profit. This month, European and Canadian authorities imposed sanctions on him and froze his assets, which include the famed Chelsea Football Club in London. The United States has not placed sanctions on him.

a pair of luxury residences near Aspen, Colo. But he also invested large sums of money with financial institutions. His ties to Mr. Putin and the source of his wealth have long made him a controversial figure.

Many of Mr. Abramovich’s U.S. investments were facilitated by a small firm, Concord Management, which is led by Michael Matlin, according to people with knowledge of the transactions who were not authorized to speak publicly.

Mr. Matlin declined to comment beyond issuing a statement that described Concord as “a consulting firm that provides independent third-party research, due diligence and monitoring of investments.”

A spokeswoman for Mr. Abramovich didn’t respond to emails and text messages requesting comment.

Concord, founded in 1999, didn’t directly manage any of Mr. Abramovich’s money. It acted more like an investment adviser and due diligence firm, making recommendations to the directors of shell companies in Caribbean tax havens about potential investments in marquee American investment firms, according to people briefed on the matter.

Paycheck Protection Program loan worth $265,000 during the pandemic. (Concord repaid the loan, a spokesman said.)

Concord’s secrecy made some on Wall Street wary.

In 2015 and 2016, investigators at State Street, a financial services firm, filed “suspicious activity reports” alerting the U.S. government to transactions that Concord arranged involving some of Mr. Abramovich’s Caribbean shell companies, BuzzFeed News reported. State Street declined to comment.

American financial institutions are required to file such reports to help the U.S. government combat money laundering and other financial crimes, though the reports are not themselves evidence of any wrongdoing having been committed.

But for the most part, American financiers had no inkling about — or interest in discovering — the source of the money that Concord was directing. As long as routine background checks didn’t turn up red flags, it was fine.

Paulson & Company, the hedge fund run by John Paulson, received investments from a company that Concord represented, according to a person with knowledge of the investment. Mr. Paulson said in an email that he had “no knowledge” of Concord’s investors.

Concord also steered tens of millions of dollars from two shell companies to Highland Capital, a Texas hedge fund. Highland hired a unit of JPMorgan Chase, the nation’s largest bank, to ensure that the companies were legitimate and that the investments complied with anti-money-laundering rules, according to federal court records in an unrelated bankruptcy case.