Hawala, a traditional money transfer system that originates from medieval Muslim traders, is flourishing in the battle against ISIS and the migrant crisis currently plaguing the EU.

The unconventional remittance network promises delivery, even to remote areas, within 48 hours after the first broker transaction. Currently, people crossing over to EU from war zones, as well as criminals issuing ransoms and participating in the black market are relying on the system to move money around.

“The ‘follow the money’ method doesn’t apply here,” said Andrea Di Nicola, a lecturer in criminology at the University of Trento. “This is a very serious problem, because the movement of money tells you a lot about how the criminal organization works.”

Unlike Western Union or bank transfers, hawala payments are untraceable. In most cases, senders are issued an unofficial note, code number or small token, for proof of the transaction. At the receiving end, the recipient can simply get the money and walk away from the deal.

Rates vary depending on the broker, usually between 3-5 percent of the transaction amount.

Most hawala brokers are business owners who are moonlighting on the side. Surprisingly, fraud is not very common in the network. The foundation of the practice draws its roots from deep ethnic or tribal relationships.

A broker who cheats customers is immediately banned from participating in the system, and the other brokers shoulder the losses to keep the transfers going.

“It’s a big problem for the police,” said Patrik Engström, head of Sweden’s border police, which has detected cases of migrants there using hawala to pay for relatives in Italy, Libya, Turkey and Iraq. “Transactions are much harder to trace—where they came from and where they went, who made them and who received them.”