Months after a prime witness went missing and two died in mysterious circumstances, the CBI has rounded up four men in connection with last year’s murder of NHAI engineer Satyendra Dubey. Calling it an ‘‘initial breakthrough,’’ CBI director U S Mishra told The Indian Express that the motive was still open. ‘‘It is only further investigations that will unravel the murder plot,’’ he said. Mishra said that the arrests were only ‘‘one dimension’’ of the Dubey case and that the agency was investigating two other angles: the deaths of Mukendra Paswan and Shivnath Sahu who were questioned by the CBI and Dubey’s complaint of corruption in his letter to the Prime Minister’s Office. As The Indian Express reported earlier, the agency has said that there was evidence to file at least four cases related to the corruption complaints Dubey made. The four arrested include Uday Mallah who is a fish vendor, Mantu Kumar who works as a porter, hawker Tutu Kumar and a PCO phone booth owner Rajesh Mehtar alias Babloo, all of them in their 20s and residents of the notorious Katari village near Gaya where Dubey was shot. Babloo’s name was first reported by