Two teenagers were held at gunpoint in a wooded area of Leyland by two older youths who demanded money from them.
The boys, aged 13 and 15, were confronted as they took a shortcut through the woodland, off Lancastergate, Leyland, at about 8.30pm on Saturday.
The 13-year-old offered one of them his mobile phone but the thieves, one of whom wore a scarf to shield his face, demanded money saying they needed it to get back to Preston.
Witnesses were interviewed by officers and CCTV footage is being scrutinised. The pair, one who was visiting the town from Huddersfield, escaped uninjured but police officers in Leyland, who believe the firearm could have been a BB gun, said they were shaken.
Detective Constable Ian Mercer said: "We've spoken to the lads and their parents and everyone is very upset by the incident.
"We're treating this very seriously we would ask anyone who thinks they may have seen anything to get in touch."
The robbers, who escaped toward the Broadfield Drive area with £5 are white, aged 17-18, and spoke with local accents.
The first is about 5ft 10ins, of medium build, and wore a white hooded top, with red writing on the front, and with blue piping around the arms and collar.
His accomplice is about 6ft, slim, and wore a dark jacket, scarf and dark tracksuit bottoms.
Anyone with information should contact Leyland CID on 01772 415846 or Crimestoppers on 0800 555 111.
l Police today launched an initiative to encourage children to hand in their BB guns at Preston Police Station, Lawson Street, Preston.
Sergeant Steve Bradshaw, of the armed response unit said it was important young people gave them to police because they can often be mistaken for real firearms.
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