Computer-savvy criminals posing as Internal Revenue Service officers at call centers in India may have bilked unsuspecting Americans out of millions, police in India said Thursday.

Police conducted a dramatic midnight raid at a call center in the country’s commercial capital of Mumbai on Tuesday and detained 770 employees for questioning. Seventy were later charged with fraud, wrongful impersonation and violating the country’s Internet safety law.

The call centers were making more than $150,000 a day through scams that took place for a little over a year, police said.

The callers told their American victims they were conducting a “tax revision” or that they had defaulted on payments to the IRS and would then obtain their personal financial information and withdraw money from their bank accounts, according to Param Bir Singh, a senior police officer in Mumbai who led the raid.

It was one of the largest such raids in recent memory, he added.

“These are the people who bring a bad name to India’s IT industry, and it is very important to weed them out,” Singh said.

In Washington, the Internal Revenue Service declined to comment on the arrests.

More than 200 police officers descended on a tall, glass-fronted office building where the nine call centers were operating during Tuesday’s midnight raid after being tipped off by neighbors.

“It looked like a regular office building and employed a lot of young people who spoke English and had a good knowledge of computers,” Singh said. “It was a very sophisticated operation they were running.”

Meanwhile, in another call center in a suburb of New Delhi, workers allegedly duped thousands of American citizens by offering to remove a virus from their computers, police said. On Sunday, police in Noida arrested six people for running a call center that sold insurance to Indians by day, but allegedly tricked American consumers by night with fake offers of tech support to correct malware and viruses.

It was not the first time that fraudulent call centers targeting U.S. citizens have been raided in India.

As more and more American businesses outsourced their back-end operations to India in the past two decades, a thriving IT industry arose here, employing millions of English-speaking software professionals. But some have also taken advantage of the trend and found ways to access data of U.S. customers and defraud them.

Call center crimes targeting Indian customers have also become a big “nuisance” for Indians, said Raj Kumar Mishra, deputy superintendent of police in Noida’s special task force. In the neighboring state of Uttar Pradesh, for example, such crimes have increased 4,300 percent in the last five years.

Most of the complaints are about online bank frauds and callers who try to access customers’ ATM identification numbers, he said. Nearly 300 people involved in such crimes were arrested in the state last year.

“There are leakages from unscrupulous bank staff who are selling the customer data illegally. That is a hole we have to plug,” Mishra said.

The state is now setting up two police stations that will handle cases of call center and online fraud exclusively.

Earlier this year, another operation was busted here that ran illegal Internet-calling exchanges in New York and Noida and offered cheap rates for callers, Mishra said.

The U.S. Federal Trade Commission reported to the Senate Judiciary Committee in June that Indian call centers, much like similar fraud operations in Jamaica, are emerging as a source of concern. The FTC said it took part this summer in a law-enforcement training program in India, “aimed at developing Indian law enforcement’s capacity to arrest and prosecute India-based individuals who perpetrate these frauds.”

Jonnelle Marte in Washington contributed to this report.