Here is how the case unfolded:
The process of allotment of 2G spectrum and licences began in August 2007. A Raja had taken over as Telecom Minister in May that year. The deadline to apply was fixed for October 1, 2007 following which 575 applications were received by 46 firms. In November, Dr Singh wrote to Mr Raja to ensure fair licence allocation. In January 2008, the Department of Telecom (DoT) issued licences on first-come-first-serve basis (FCFS).
However, a year later, the Central Vigilance Commission (CVC) directed the CBI to investigate the allegations of illegalities in the allocation of 2G spectrum. In October 2009, the CBI filed an FIR against unknown officers of DoT, unknown private persons/companies and others.
The centre and Mr Raja were given 10 days in September 2010 by the Supreme Court to respond to three pleas alleging a Rs 70,000-crore scam in the grant of telecom licences.
Two months later the CAG submitted its report on 2G spectrum claiming losses of Rs 1.76 lakh crore to the exchequer which followed Mr Raja's resignation. Mr Raja, former Telecom Secretary Siddhartha Behura, Mr Raja's erstwhile private secretary Ravindra Kumar Chandolia and Shahid Usman Balwa, promoter of Swan Telecom Pvt Ltd were arrested by the CBI in February 2011. The former telecom minister spent nearly a year in prison before getting bail.
On April 2, 2011, the CBI filed its first charge sheet and named Mr Raja, Mr Chandolia and Mr Behura as the main accused. Reliance ADAG Group Managing Director Gautam Doshi, its senior Vice President Hari Nair, Group President Surendra Pipara, Swan Telecom Promoters Shahid Usman Balwa and Vinod Goenka and Managing Director of Unitech Ltd Sanjay Chandra were also made accused in the case. Three companies -- Reliance Telecom Ltd, Swan Telecom Pvt Ltd and Unitech Wireless (Tamil Nadu) Pvt Ltd - were also included in the charge-sheet.
In its second charge sheet, it named DMK chief M Karunanidhi's daughter and lawmaker Kanimozhi and four others. Charges framed against all 17 accused in October 2011. Trial in the case began in November.
Special Judge OP Saini, who is exclusively dealing with the cases arising out of the 2G spectrum scam, has been hearing three cases- two filed by the CBI and the third by the Enforcement Directorate (ED).
The second case involved Essar Group promoters Ravi Ruia and Anshuman Ruia, Loop Telecom promoters Kiran Khaitan, her husband IP Khaitan and Essar Group Director (Strategy and Planning) Vikas Saraf. Three firms, Loop Telecom Ltd, Loop Mobile India Ltd and Essar Tele Holding Ltd (ETHL) were also named in the charge sheet.
The ED had filed a charge sheet in April 2014 against Mr Raja, Ms Kanimozhi, Shahid Balwa, Vinod Goenka, Asif Balwa, Rajiv Aggarwal, Karim Morani, Sharad Kumar and others in connection with a money laundering case relating to the scam.
In its charge sheet, the ED named DMK supremo M Karunanidhi's wife Ms Dayalu Ammal as accused and alleged that Rs 200 crore was paid by STPL promoters to DMK-run Kalaignar TV.