Fake Beyonce tickets have been sold to unsuspecting fans who were duped out of thousands of pounds.
British Transport Police said more than 140 sets of the bogus tickets for concerts at the Phones 4U Arena in Manchester were sold for £100 each online.
So far seven victims have come forward but the force said many more sets had been sold with buyers from across the UK paying for at least two tickets each.
Detective Constable Mike Dermody said: "It seems that someone has been selling fake tickets to concert goers who only realise the tickets aren't genuine when they get to the venue.
"We believe there have been more than 140 sets of fake tickets sold, though only seven victims have come forward so far.
"In most cases the tickets have sold for at least £100 each and each victim has bought at least two tickets, so we are potentially looking at a fraud worth more than £28,000."
He appealed for more victims to come forward.
"Each transaction has taken place over the internet, using public selling websites, with the victims arranging to meet someone in Manchester to pick up the tickets," the detective added.
"I am, therefore, appealing to anyone who has been sold fake tickets, but has not yet spoken to police to come forward.
"We have a number of leads, but need as many people as possible to get in touch."
Anyone with information can call Action Fraud on 0300 123 2040 or British Transport Police on 0800 40 50 40.