A masked raider held a gun to the head of a woman shop worker as he demanded cash from the till.
The 56-year-old victim, who has worked at the Oxford shop for several years, was left terrified by her ordeal.
Delteys supermarket in Blackbird Leys Road was held up by two men at about 6.45pm on January 19.
A white man, who covered his face with a scarf, held a gun to the woman's head and ordered her to hand over cash. The offender then reached over to grab a handful of £10 notes from the till.
Detectives are linking the robbery with a violent incident involving a knife and hammer at Acorn News, St Nicholas Road, Littlemore, at about 6.05pm on the same day.
The shop owner was struck with a hammer, suffering injuries to his arm. He and two women serving in the shop fought back and managed to pull a hat from one of the offenders, who dropped the hammer. Both women were punched.
Three men, one black and two white, with their faces covered and wearing hooded tops are being sought by police. Nothing was taken.
The gun-wielding offender in the Delteys robbery wore a hooded top and was in his late teens or early 20s. He was accompanied by a black youth in his teens.
Dc Bill Walsh, of Oxford CID, said: "We would appeal to witnesses or anyone with information to come forward."
Call police on 08458 505505 or the anonymous Crime- stoppers line on 0800 555111 with information .
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