The Central Bureau of Investigation (CBI) launched a coordinated search operation at over 10 locations in multiple cities on Saturday in connection with the NSE co-location scam case, officials said.

The search operation will cover brokers at more than 12 premises in Mumbai, Gandhinagar, Delhi, Noida, Gurugram and Kolkata, among other cities, they added.

The central agency has filed a chargesheet against former National Stock Exchange (NSE) CEO and MD Chitra Ramkrishna and group operating officer Anand Subramanian in the case, the officials said.

The probe has so far established that from 2010 to 2015, when Ramkrishna was managing the affairs of the NSE, OPG Securities, one of the accused in the FIR, had connected to the secondary POP server on 670 trading days in the "Futures and Options" segment.

The CBI has kept the probe open into allegations of preferential access granted to certain brokers by NSE officials and undue gains made out of it during the tenure of Ramkrishna and Subramanian.

Ramkrishna, who succeeded former CEO Ravi Narain in 2013, had appointed Subramanian as her advisor, who was later elevated as the group operating officer (GOO) at a fat paycheque of Rs 4.21 crore annually, the officials said.

Subramanian's controversial appointment and subsequent elevation, besides crucial decisions, were guided by an unidentified person, who Ramkrishna claimed was a formless mysterious "yogi" (mystic) dwelling in the Himalayas, a probe into her e-mail exchanges during a SEBI-ordered audit had shown.

The central probe agency had booked stock broker Sanjay Gupta, the owner and promoter of Delhi-based OPG Securities Private Limited, in 2018 for allegedly making gains by getting early access to the stock market trading system, the officials said.

The agency is also probing unidentified officials of the Securities and Exchange Board of India (SEBI), NSE, Mumbai and other unidentified people.

"It was alleged that the owner and promoter of the said private company abused the server architecture of the NSE in a conspiracy with unidentified officials of the NSE.

"It was also alleged that unidentified officials of NSE, Mumbai had provided unfair access to the said company using the co-location facility during the period 2010-2012 that enabled it to log in first to the exchange server of the stock exchange that helped to get the data before any other broker in the market," the CBI has alleged in the FIR.