Police in the western Indian city of Thane have arrested more than 750 people suspected of defrauding US citizens from a fake call centre.
Officers say the suspects obtained lists of US tax defaulters and used threats to obtain their bank details.
The scam is said to have netted more than $150,000 (£118,000) a day, making it one of the biggest frauds in India’s history.
Thane police officials said they were contacting the FBI to ask for help.
Some 70 of those detained were formally arrested, police said, with the rest released pending further inquiries. Nine people believed to have led the scam have been identified.
The scammers pretended to be members of the US Internal Revenue Service, and told victims they owed back taxes.
In some cases, the victims were duped into buying gift vouchers from different companies and bullied into revealing voucher ID numbers, police said.
The scammers then made purchases with the voucher numbers.
Police say those involved in the scam at the Indian end retained 70% of the earnings, with 30% going to their US collaborators.
Paramvir Singh, the police commissioner of Thane, told reporters that 851 hard disks, high-end servers, and other electronic equipment had been seized.
Mr Singh said overnight raids on Wednesday had lasted well into the morning, and involved more than 200 policemen who had raided buildings in three locations in the city.
Thane police superintendent Mahesh Patil told BBC Hindi that the investigation could open up cases from other countries as well.
A US State Department official said: “We have seen reports and are following the situation closely to confirm any US citizen involvement. We would refer you to the local Indian authorities for further details on the case.”