As scrutiny of her business empire increased, African billionaire Isabel dos Santos turned to one of the world’s most secretive tax havens to conceal a $1.8 million luxury apartment in the Portuguese capital, Lisbon.

The haven in question? The United States.

Dos Santos and her husband owned the apartment through a company in Delaware called Decade International LCC, according to business records and emails obtained through Luanda Leaks, an International Consortium of Investigative Journalists investigation.

While the Luanda Leaks documents don’t say why dos Santos formed a company in Delaware, the U.S. state is one of the easiest places in the world to set up a shell company, and offers owners of those companies nearly complete anonymity. And it is only getting more opaque.

In its annual “financial secrecy index,” released last week, the Tax Justice Network ranked the U.S. as the world’s second-most secretive jurisdiction, behind only the Cayman Islands. (Other U.S. states also host murky shell companies, but Delaware is far and away the biggest haven).

“The U.S. has actively bucked the global trend towards greater transparency, increasing the range of secrecy vehicles on offer,” Alex Cobham, the advocacy group’s chief executive, told ICIJ.

Luanda Leaks reporting found that dos Santos funneled hundreds of millions of dollars in illicit wealth from her home country of Angola to shell companies in secrecy havens. Once offshore, she used the cash to buy up businesses and assets, including luxury homes like the one in Portugal.

Angola froze more than $1 billion of her assets in December and Portugal froze dos Santos’ bank accounts in February. Dos Santos denies wrongdoing.

The dos Santos business records were obtained by the Platform to Protect Whistleblowers in Africa, and shared with ICIJ. The documents include almost 100 emails, invoices, electricity and sewage bills and property tax assessments that describe the Lisbon property and the Delaware company.

The apartment is in a high-rise building on Lisbon’s Avenida António Augusto de Aguiar, above the city’s largest shopping mall and next to a lush, landscaped public park.

In a draft contract prepared in July 2012, dos Santos and Decade International were listed as buyers and a Maltese company the sellers. The draft sale price was $1.8 million. The final purchase documents are not part of Luanda Leaks and the final price and terms are unknown.

The following year, dos Santos appears in control of Decade International as it pays bills and carries out renovations on the Lisbon apartment. In 2014, dos Santos and her husband spent $408,000 remodeling and on bathroom renovations, including $7,165 on heated towel racks, according to an invoice sent to the Lisbon apartment.

Through lawyers in London, dos Santos did not address questions about her use and control of the Delaware company.

Decade International was first registered in 2003 by a Delaware incorporation services company based at North Orange Street in Wilmington. The company later relocated, on paper, to the mailbox of another incorporation specialist in Dover, Delaware.

Luanda Leaks documents show dos Santos in control of the Delaware company. Emails show that dos Santos signed to approve bank transfers on behalf of Decade International and in 2013, advisers hand-wrote “IS/SD account” on the top of a 2013 water bill addressed to Decade International, referring to the couple’s initials.

Delaware does not publish information about the owners or shareholders of companies incorporated in the state. U.S. enforcement authorities, including the FBI, require a subpoena to access the full corporate records of a company such as Decade International.

“The fact that Isabel dos Santos had a Delaware company among her 400 or so related companies is a perfect example of why so many nonprofits are pushing for transparency in U.S. corporations,” said Debra LaPrevotte, a former anti-corruption special agent at the FBI and a senior investigator with The Sentry, which exposes kleptocrats and human rights abuses. “A property purchased or a deal conducted with a U.S. corporation adds an air of legitimacy to the deal.”

U.S. Secretary Mike Pompeo, speaking in Angola last week, told reporters that the United States will help Angola “repatriate capital illegally domiciled abroad.” When it comes to opaque financial transactions, Pompeo said, “the U.S. uses its resources to correct what is wrong.” ‘

The Federal Bureau of Investigation and Department of Justice declined to comment on whether they were investigating Decade International.

Clark Gascoigne, executive director of the FACT Coalition, which advocates for corporate transparency, said that he is optimistic a bipartisan bill to end the abuse of anonymous companies will soon pass in the U.S. Senate. The bill, known as the Corporate Transparency Act, would require many forms of limited liability corporations, like Decade International, to disclose their real owners. The White House backs the bill, which passed the House of Representatives last year.