Via call centres, the victims had been convinced to invest large sums in shares and foreign currencies, among other things, police said on Wednesday.
Sometime later, however, the victims found out that no corresponding transactions had been carried out for the money.
A company in Petah Tikva near Tel Aviv is at the centre of the investigation.
Eleven other people were released after questioning, it said.
In addition, accounts with about €1.1 million ($1.3 million) had been frozen.