A Worthing shopkeeper has been robbed at gunpoint in the town's 11th armed raid since Christmas.
Pickwick News in Brighton Road became the latest victim on Friday evening of a string of robberies.
But officers are not linking the latest robbery with nine others which are thought to be the work of the same shotgun-toting crook.
Armed police were at Pickwick News within minutes of Friday's raid at 9pm but the gunman still escaped with £500 cash.
However, officers did arrest a 22-year-old at the scene in connection with an unrelated offence.
Police spokesman Nick Sandford said: "There is no real reason why we are suddenly having this problem.
"It's as much a mystery to us as it is to you at the moment. Our investigation is ongoing and we will be going from there."
The offender was about 35, white, 5ft 8in tall and wore a khaki jacket with a beige hood and blue tracksuit bottoms.
Meanwhile, police working on Operation Texan, the hunt for the man thought responsible for nine of the recent robberies, are coming under pressure from retailers.
The raider has threatened staff in petrol stations, off-licences and general stores around Worthing with a shotgun.
His targets have included the Murco garage in Sompting Road on December 28 and the Heene Road Texaco garage on January 4 and again on January 8.
Thresher in Church Road, Tarring, was hit on January 17 and again on February 4. The nearby Village Stores were raided on January 29.
The BP Garage in Brighton Road was targeted on January 13 and the Spar in Heene Road was hit on February 8.
The raider is white and aged between 20 and 30. He has worn a white-hooded, light-coloured top with a blue jacket over the top.
Police have warned the public not to approach him.
Anyone with any information should contact Detective Constable Amanda Lawrence on 0845 6070999, ext 81256.
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