Judith Ducker, 64, is believed to have been attacked during the robbery at Wellgate News, on Wellgate, in Rotherham, in September.
Mrs Ducker, who had underlying health conditions, was taken to hospital with head injuries and died on Thursday.
Further tests are due to be carried out after results from a post-mortem examination were inconclusive and did not reveal a cause of death.
South Yorkshire Police are now appealing for a man and a woman seen on CCTV around the time of the robbery, between 11am and 11.25am on Thursday September 1, to come forward.
Detective Sergeant Andy Shields said: “This is an incredibly sad time for Mrs Ducker’s family and friends and my thoughts and sympathies are with them.
“We are continuing to investigate the incident and we are very keen to identify and trace the man and woman on the CCTV images, as we believe they may have entered the store between the times stated and be the last people to have possibly seen or spoken to Mrs Ducker prior to the alleged attack.
“I want to make clear they are not in any trouble at all and we do only want to trace them as potential witnesses, as they may have vital information that is crucial to our investigation.”
“If you believe this is you in the picture, or you know who the man and woman may be, please call police on 101, or Crimestoppers anonymously on 0800 555111, quoting incident number 388 of 2 September 2016.”
:: Paul Storm Blueitt, 36, of Cambridge Crescent, Rotherham, has been charged with attempted murder and robbery and has been remanded in custody.