The Supreme Court on Tuesday asked the CBI to investigate the murders of activists Narendra Dabholkar, Govind Pansare, M.M. Kalburgi and Gauri Lankesh if it found a "common thread" in the crimes.The Central Bureau of Investigation (CBI) is already investigating the Dabholkar murder case.As the investigating agency wanted some time to look into the matter, a Bench of Justice Uday Umesh Lalit and Justice Navin Sinha gave the agency time till the first week of January to come back with its response.The court asked the CBI if there was any common link in the murders of the activists. It raised the question while hearing a plea by Umadevi Mallinath Kalburgi, the widow of M.M. Kalburgi, who is seeking an SIT probe into his murder.On November 26, while hearing Umadevi's plea, the apex court had pulled up the Karnataka Police for not doing anything to probe the murder of Kalburgi, who was shot dead outside his house in Dharwad on August 30, 2015."What have you done so far? You are just fooling around," the Bench of Justice Fali Nariman and Justice Navin Sinha had said, as it asked the state police to spell out how much time it would require to complete the investigation.