THREE weeks after the Rs 133-crore fertiliser scam broke, investigators dealing with the case believe any serious headway is possible only with the arrest and systematic interrogation of Prabhakar Rao. But as a sequence of events over the fortnight indicate, the arrest is being thwarted at the highest levels in the CBI. Sources say that in the third week of June, CBI Joint Director S. Gopalachari, heading the probe, told his superiors about his intent to pick up the former prime minister's son for questioning, making it clear that it would be a regular judicial arrest. Shortly, a team of officials met him and conveyed CBI Director K. Vijaya Rama Rao's opposition to this.