Swedish police arrested four people in the Stockholm area on Tuesday on suspicion of money laundering and local media reported the authorities believed the money had been sent to Islamic State militants in Syria, Reuters reported.
The Swedish Economic Crime Authority said money laundering was suspected in six companies but further details could not be provided while the investigation was under way.
Citing unnamed sources, Swedish daily Dagens Nyheter reported the state prosecutor believed the suspects, three men and one woman, sent money to Islamic State. The police operation was described as coordinated and well-planned.
"The crime includes operations in six companies and the raid was prompted by suspicions about accounting fraud in all six companies," the agency said in a statement, adding the firms were located in the suburbs of Stockholm.
The Economic Crime Authority could not be reached for a comment.
Swedish authorities are preparing a law this year to criminalize both travel to take part in foreign conflicts and the organizing and funding such travel.