Sohel Daud, a senior agent of the Federal Bureau of Investigation (FBI), who arrived here this morning, met senior officers, including Police Commissioner Param Bir Singh, at the Police headquarters here.
The American security and crime-fighting agency came into picture after it emerged that the victims of the call centre racket were US citizens.
The fraud is of a big magnitude which cannot be estimated at this stage, he said.
"We will verify the information which the FBI has shared with us. It will help in our investigation. The FBI team also appreciated our efforts (in uncovering the scam)," Joint Commissioner of Police Ashutosh Dumbre told PTI.
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The FBI officer will be in the city for a few more days and question those arrested in connection with the scam, police sources said.
Crime Branch teams are also looking for Thakkar and authorities at the international borders, airports and sea ports have been alerted to thwart his attempt to leave the country, Dumbre said.
Police unearthed the scam, involving more than Rs 500 crore, last week after raiding seven call centres in Mira Road locality. The racket had been going on for a few months, he added.
Police have so far arrested 71 people in connection with the case. Some of the call centres involved in the racket were run illegally from the premises of Hari Om IT Park, Universal Outsourcing Services and Oswal House on Mira Road.
(Reopens BOM 24)
Meanwhile, a magistrate's court in Thane extended the police custody of eight of the accused today.
Assistant Commissioner of Police Mukund Hatote gave this information to the court while seeking further police custody for eight of the 13 accused.
These eight accused were members of the core group of the conspirators, he said.
The court remanded them in police custody upto next Monday.