The tri-border area, where Paraguay, Brazil and Argentina meet, is a lawless region where drugs trafficking, gun running and counterfeit goods are rife.
The BBC has now found documents showing the suspicious transfer of large sums of money to the Middle East, which investigators believe goes to fund terrorism. The BBC's Andrew Bomford reports from Ciudad del Este in Paraguay.
It didn't look like the global centre of a business sending billions of dollars overseas, but on the first floor of the dingy-looking shopping arcade, if you could get past the two guards blocking the stairs, there it was - a shop, looking like a pawnbrokers, called Telefax.
Kassem Hijazi denies allegations of terrorist financing and tax evasion
According to Paraguayan and American investigators, Telefax, owned by a Lebanese businessman called Kassem Hijazi, is responsible for transferring huge sums of laundered money overseas and hiding the identities of the people responsible.
The money is believed to be the proceeds of crime - anything from drug smuggling, to gun running, to counterfeit goods to tax evasion.
"From the evidence and documentation we saw, it was clear that this man was moving large sums, hundreds of millions of dollars, through its doors, in its own name, hiding the identities of who was truly the owners of the money," said Carlos Maza, of the US Department of Homeland Security.
"Kassem Hijazi is a serious player who more than anything else has found the vulnerability in the Paraguayan system, the ability to control how money is moved through its banking system."
But US investigators are particularly worried that some of this money goes to fund terrorism as well as militant organisations like Hezbollah and Hamas.
Robert Morgenthau, the New York District Attorney, has prosecuted a number of American banks for moving millions of dollars from the tri-border area to what he suspects are terrorist bank accounts in the Middle East.
"We've found money going to the Arab Bank in Ramallah," he told the BBC.
"The Arab Bank is well known as one of the banks used by terrorist organisations. But that's part of the frustration. You don't know who's sending the money and you don't know who's receiving it."
The BBC saw company accounts for Telefax showing a business with an annual turnover of just $50,000.
But a large number of money transfer documents, obtained in a series of raids by Paraguayan prosecutors, show Telefax making international transfers worth ten times that amount almost on a daily basis.
The owner of Telefax, Kassem Hijazi, agreed to do an interview with the BBC.
He produced a large amount of the prosecution paperwork allegedly showing thousands of money transfers, but claimed that every single one of the documents had been forged.
"The proof is here," Mr Hijazi said, indicating the transfer documents.
"They have to prove that I've done it. Even the prosecutor says the documents are false, not us, the prosecutor. I wasn't transferring money abroad. It's the money exchange houses that send the money, and they've forged the documents. We don't do transfers abroad."
Mr Hijazi did admit to using numbers instead of names for his clients, effectively hiding their identities, but he said that was merely to make his paperwork easier.
30,000 people a day cross the bridge between Paraguay and Brazil
The BBC examined a number of the transfer documents and saw large amounts of money, around $10m, moving to Lebanon in the space of a year.
Three transfers, for $100,000, $70,000, and $42,200, went in the space of two days to companies in Beirut which did not appear to exist.
Adolfo Marin, the original prosecutor in the case, said it was very difficult to investigate the money transfers because the banks in Beirut were dominated by Hezbollah.
"I have no idea what they can export to us from Lebanon, so necessarily the money that goes to Lebanon is not for imports," he said. "So it is possible to formulate a hypothesis about the probability of money laundering and links with terrorism."
Kassem Hijazi strongly denied any involvement in terrorist financing. "It's absurd," he said.
"It doesn't happen here. The people I work with are friends, not terrorists. They've been investigated and if there was some evidence they would have been charged."
'Helping our brothers'
This view was supported by Sheik Taleb Jomha, the Muslim leader of the 30,000-strong Lebanese community in the tri-border area.
The Paraguayan military patrol the Parana River looking for smugglers
"I am not telling you a secret when I say that Iran and Syria are supporting Hezbollah," he said.
"Iran has the ability to send weapons and rockets, not us. They say money is moving from here to the Middle East. That's right. But not to help political or military groups, but to help our brothers and sisters who need help."
Kassem Hijazi is not facing charges of money laundering or even terrorist financing.
In Paraguay, funding terrorism is not a crime, and the law on money laundering is out of date, making it difficult to achieve a prosecution.
A new law has been languishing, unapproved, in the country's Congress for more than two years.
In the meantime, Mr Hijazi has been accused of tax evasion, which he also denies, and is expected to face trial in 2007.