Panicking, she rang the number and told a man who said he was from the IRS: ‘I can pay $500,’ half the sum demanded. "I could do a payment plan. I just can't pay all of it at once." "Ma'am, you can pay $500 today itself. You can do that?" the man asked, adding that lawyers would look at her accounts and work out a monthly payment plan, but she had to pay half now.

In transcripts of the conversation that investigators shared with agency, the man told her to keep the phone line open and drive to a nearby grocery store, where she bought $500 worth of iTunes gift cards and gave the 'agent' the redemption codes.

She had just been scammed - one of at least 15,000 people the US Justice Department says lost more than $300 million in an ‘enormous and complex fraud’ running since 2013. The department last month brought grand jury charges against 56 people in India and the United States for ‘telefraud’scams run from fake call centers in India.

Investigators have arrested 20 people in the United States, and Indian authorities have made 75 arrests following October raids on three premises in the Thane suburb of Mumbai. Charges include conspiracy to commit identity theft, impersonation of an officer of the United States, wire fraud and money laundering.

Indian police say they are looking for Sagar Thakkar, a man in his early 30s also known as Shaggy, who they believe masterminded the scam. Thakkar was also among those named by the US Department of Justice. Agency was unable to contact Thakkar for comment; he is not known to have a lawyer, and police believe he fled to Dubai last month.

"We are trying to complete the procedure to issue a red corner notice for Thakkar," Parag Manere, a deputy commissioner at Thane police, said, referring to an Interpol arrest warrant.

Police said Thakkar led a lavish lifestyle, frequenting 5-star hotels and driving expensive cars with proceeds from the scam. He gave one, a 25 million rupee ($365,000) Audi R8, to his girlfriend. "We have seized an Audi car, and are trying to find other assets of Thakkar," Manere said.

The FBI, which is involved in the investigations, declined to comment. The Department of Justice did not respond to requests for comment for this article. At a news conference last month, Assistant Attorney General Leslie Caldwell said the US would seek the extradition of suspects in India, and warned others engaged in similar schemes they could face jail terms.

In interviews before the US charges were filed, police, suspects and call center workers in India told agency how the scam was run. Training materials and taped conversations, which investigators believe were made by call center instructors for training purposes, shed some light on an operation aimed to exploit the aged and gullible.