Boston. In what could be a major break in the case of the Boston Marathon, investigators are on the hunt for a man seen in a department store surveillance video dropping off a bag at the site of the bombings that killed three people, officials said.Separately, a law enforcement official confirmed that authorities have found an image of a yet-to-be-identified potential suspect.Meanwhile, the third victim was identified as Boston University graduate student Lu Lingzi by The Shenyang Evening News, a state-run Chinese newspaper.Wednesday’s developments on the attack that left more than 170 injured at the world’s oldest and most prestigious marathon, is a possible turning point in a case that has investigators analysing photos and videos frame by frame for clues to who carried out the twin bombings and why.Surveillance footageCity Council President Stephen Murphy, who said he was briefed by Boston Police, said investigators saw the image on surveillance footage got from a department store near the finish line, and matched the findings with witness descriptions of someone leaving the scene."I know it’s very active and very fluid right now that they are on the chase,” Murphy told The Associated Press. "They may be on the verge of arresting someone, and that’s good.”The bombs were crudely fashioned from ordinary kitchen pressure cookers packed with explosives, nails and ball bearings, source close to the case said.Investigators suspect the devices were then hidden in black duffel bags and left on the ground. As a result, they were looking for images of someone lugging a dark, heavy bag.One department store video "has confirmed that a suspect is seen dropping a bag near the point of the second explosion and heading off,” Murphy said.A law enforcement official, who spoke to AP on condition of anonymity, confirmed that investigators had an image of a potential suspect whose name was not known to them.