More than 125 forged banknotes have been turned into Stockholm police since the start of the year.
"I think we're just seeing the tip of the iceberg," detective C.G. Wrangel of the Stockholm police told the TT news agency.
Among the fake notes turned into the police are 50, 100, and 500 euro notes, as well as several 20-kronor bills.
Even a forged Chinese yuan note has been discovered.
Forensic experts have said the fake 20-kronor notes are easy to spot because the forgeries aren't particularly well made.
"The best advice to avoid being duped is to get in the habit of feeling the bills. The paper of real bills has a certain roughness, while the fake 20-kronor notes are smoother," Michael Johansson, a bank note specialist with Swedish National Laboratory of Forensic Science (Statens kriminaltekniska laboratorium, SKL), told TT.
The false 20-kronor bills also lack a watermark and a metal security thread, features that are easily discovered when the bill is held up against the light.
Johansson explained that forging lower-denomination bills may be a strategy to avoid detection.
He also doubted any connection between the forged euro notes and the faux kronor bills.
"The euro is attacked by professional gangs that produce fake bills in large volumes. Then they go spread them out in different countries," Johansson said.
"There are very well-made forgeries of euro notes and there is reason to be on your guard."
Police have now launched a preliminary investigation into bank-note forgery following the recent fake-bill find.
"We're working on finding whoever is behind this, but it's hard to connect the bills to one perpetrator," Wrangel said.
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