Thieves escaped with more than £13,000 after they raided a convenience store armed with a meat clever and a metal bar in Hoghton, say police.
And a male member of staff was injured in what has been described as an "extremely violent" robbery after he was head-butted.
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Detectives say that three men wearing balaclavas entered the Spar on Alder Drive at around 9.50pm on Tuesday, October 17.
The men forced one of the shop workers to empty the till and also seized a quantity of cigarettes and alcohol.
The thieves then ordered another male worker into an office area to open a safe. They stole £13,000 from it and made off out of the shop.
DS Chris Walsh from Preston CID said: “This was an extremely violent incident where a shop employee, just doing his job, was badly assaulted. Both he and the other worker have been left extremely shaken up by their ordeal.
“We think that the store was targeted and I would like to reassure people that there is not thought to be any wider threat to residents in the area, however, we have stepped up police patrols.
“If you have any information about the incident, please contact us as soon as possible.”
One of the offenders is described as white, around 6ft and of medium build. Another is described as black, around 5ft 9ins, of medium build with a moustache. The last offender is described as white, around 5ft 7ins and was of slim build. All three were wearing dark clothing and balaclavas.
Anyone with information can call Preston CID on 01772 209749 or if they fail to get an answer, 101, quoting incident reference 1585 of October 17.
Alternatively, independent charity Crimestoppers can be contacted anonymously on 0800 555111 or online at crimestoppers-uk.org.