What is the co-location case for which ex-NSE CEO Chitra has been arrested?

The NSE co-location case refers to a series of events wherein some brokers received preferential access through the co-location facility, resulting in faster access to the markets. The matter came to light after a whistleblower wrote to SEBI in 2015. The NSE co-location facilities were launched in 2009 while Chitra Ramkrishna was appointed as the Joint MD the same year.