For wealthy Americans looking to veil their assets and shield some of their income from taxation, there is no need to go to Panama or any other offshore tax haven. It’s easy to establish a shell corporation right here at home.

“In Wyoming, Nevada and Delaware, it’s possible to create these shell corporations with virtually no questions asked,” said Matthew Gardner, executive director of the Institute on Taxation and Economic Policy, a nonprofit research organization in Washington.

In some places, it can be more difficult to get a fishing license than to register a shell company. And it doesn’t cost much more.

The Panama Papers — the cache of leaked documents from a Panama law firm, Mossack Fonseca — have revealed how thousands of the firm’s clients, including an array of powerful figures around the world, stashed billions of dollars in tax havens. So far only a tiny number of American names have surfaced (although that could change as more of the documents are reviewed).

That in no way means that Americans citizens are refraining from such practices, experts emphasized.

“This is just one firm in one place,” said Gabriel Zucman, an economist and the author of “The Hidden Wealth of Nations: The Scourge of Tax Havens,”“So it cannot be representative of what’s happening as a whole in the world.”

But Mr. Zucman, who estimates that about 8 percent of the world’s financial wealth — more than $7.6 trillion — is hidden in offshore accounts, said another reason was that it is so simple to create anonymous shell companies within the United States.

Wealthy individuals and businesses that want to mask their ownership can conveniently do so in the United States, and then stash those assets abroad.

Yet while the United States demands that financial institutions in other countries share information about Americans with accounts overseas, its reciprocation efforts fall short, critics say.

“You see a ton of wealth in tax havens in Switzerland and the Cayman Islands that is owned by shell companies that are incorporated in Panama or in Delaware,” he said. “The bulk of this wealth does not seem to be duly declared on tax returns.”

A recent report by the Institute on Taxation and Economic Policy called“Delaware: An Onshore Tax Haven” noted that the state’s lack of transparency combined with an enticing loophole in its tax code “makes it a magnet for people looking to create anonymous shell companies, which individuals and corporations can use to evade an inestimable amount in federal and foreign taxes.”

Delaware allows companies to shift royalties and similar revenues where they actually do business to holding companies in Delaware, where they are not taxed.

Heather A. Lowe, the legal counsel and director of government affairs for Global Financial Integrity, a research and advocacy group in Washington, warned that the problem was much more widespread than just a handful of states.

“You can create anonymous companies anywhere in the United States,” Ms. Lowe said. “The reason people know about Delaware, Nevada and Wyoming is because these states market themselves internationally.”

In annual reports, Delaware’s secretary of state has boasted of marketing efforts that have “helped the state reach thousands of legal professionals in dozens of countries across the globe” and visits by delegations to Brazil, Israel, Canada and Spain “to tell the Delaware story.” The resulting incorporation fees make up a hefty portion of state revenue.

Although individuals and businesses can have legitimate reasons to want to screen their holdings — for privacy or to prevent competitors from discovering investment plans — several experts said cloaking wrongdoing was a more common purpose.

Aside from avoiding taxes, shell companies are routinely used by terrorist organizations to hide assets, by political donors to sidestep campaign finance laws and by criminals to launder money, Mr. Gardner said.

The Treasury Department indicated this week that it planned to require financial institutions to verify the identities of customers who set up accounts in the names of shell companies, thus closing a loophole in the American banking system that thwarts transparency efforts.

The Treasury also recently began a program that tracks people who use shell companies to purchase expensive real estate in New York and Miami.

But the new rules would not affect state law.

John A. Cassara, a former special agent for the Treasury Department, said that American and foreign law enforcement officials conducting investigations were regularly stymied by state secrecy laws surrounding shell corporations.

“If somebody is conducting an investigation and it comes back to a Delaware company and you want to find who or what is behind that company, you basically strike out,” he said. “It doesn’t matter if it’s the F.B.I., at the federal level, state or local. Even the Department of Justice can’t get the information. There is nothing you can do.”

He recalled a case where investigators ended up abandoning their inquiry of a Nevada-based corporation that had received more than 3,700 suspicious wire transfers totaling $81 million over two years.

“Why incorporate in Nevada?” the state’s website advertised in 2007. “Minimal reporting and disclosing requirements. Stockholders are not public record.”

Mossack Fonseca, the Panama law firm, set up offices in Nevada and Wyoming.

After revelations came to light about Americans using Swiss bank accounts to evade taxes, the United States in 2010 passed the Foreign Account Tax Compliance Act, which requires financial firms in other countries to disclose details about American clients with offshore accounts.

Yet the United States is one of the few countries that has refused to sign new international standards for exchanging similar financial information with other countries.

Another country that has failed to sign the standards? Panama.

“The United States demands that the rest of the world tell it when an American holds an account at a foreign institution, but the U.S. does not return the courtesy by automatically providing comparable information on foreign investors in U.S. banks to their home tax jurisdictions,” said Edward D. Kleinbard, a law professor at the University of Southern California and a former chief of staff of Congress’s Joint Committee on Taxation.

Mr. Kleinbard and Ms. Lowe of Global Financial Integrity said that American banks are awash in money from foreign investors using shell corporations to conceal money from their own governments and law enforcement.

“Where is that money going?” Ms. Lowe asked. “Not to Delaware, Nevada and Wyoming, but New York, Miami and Los Angeles banks.”

Originally published by Patricia Cohen for The New York Times.