CBI has made substantial progress in the investigation into the NSE co-location case where markets regulator SEBI has found violations of norms by the stock exchange as well as some of its former top level employees, Finance Minister Nirmala Sitharaman said on Tuesday.The government and SEBI have received some complaints in the matter of NSE co-location.In a written reply to Rajya Sabha, the minister said that during the investigation into the matter, it was noticed that certain stock brokers got preferential access to the trading system. "CBI has also registered a case in the matter in 2018 under relevant sections of India Penal Code, 1860, Prevention of Corruption Act, 1988 and Information Technology Act, 2000. The CBI investigation is still going on. CBI has informed that substantial progress has been made in the investigation of the case," Sitharaman said.The finance minister also said that for lapses in this regard, NSE's former CEOs and top officials have been held responsible for breaches of the relevant provisions of Securities Contracts (Regulations) (Stock Exchanges and Clearing Corporations) Regulations, 2012 and monetary penalty and other restrictions have been imposed on them.Former MD & CEO of NSE Chitra Ramakrishna is currently under the custody of CBI along with her colleague and Group Operating Officer (GOO) during her tenure Anand Subramanian.The arrest pertaining to the co-location case follows an FIR, which was registered in May 2018, amid fresh revelations about irregularities at the country's largest stock exchange. Before Chitra Ramakrishna, Ravi Narain was the MD & CEO of the exchange.Also Read: Fixed trade margins of over 40 costly cancer drugs, price reduced: MandaviyaIn February this year, SEBI imposed monetary penalty on NSE as well as Ramakrishna and Narain, and two other officials for lapses in recruitment at the senior level. Narian was at the helm of affairs at NSE from April 1994 till March 2013, while Ramakrishna was MD & CEO of the stock exchange from April 2013 to December 2016.SEBI has observed that NSE and its top executives violated norms in appointing Subramanian as GOO and advisor to the MD. In a separate reply, Minister of State for Finance Pankaj Chaudhry said, "SEBI has carried out thorough examination/investigation of the complaints and has taken suitable actions against various entities/individuals." SEBI received some complaints in 2015 alleging certain irregularities in respect of co-location facility provided by NSE, he said.Further, SEBI has advised stock exchanges to develop suitable surveillance mechanism adopting technology tools. SEBI has also empanelled external forensic auditors to assist the investigating authority, Chaudhry added. "SEBI and the stock exchanges have put in place necessary systems and practices to promote a safe, transparent and efficient market and to protect market integrity."In this regard, SEBI and the stock exchanges have surveillance mechanisms in place to track the activities on the stock exchange platform and to generate alerts based on dynamic, system-based parameters," the minister said. He was replying to a question by Shiv Sena MP Priyanka Chaturvedi on whether the government has taken measures to ensure that stocks are not manipulated and preferential stock trading does not take place.