Following several months of surveillance, police on Tuesday arrested a 30-year-old man suspected of being the operation’s ring leader, according to the Dagens Nyheter (DN) newspaper.

The arrest comes days after police confiscated counterfeit bills worth “millions” in Swedish kronor during a raid on a site in Märsta north of Stockholm.

Five other men with criminal backgrounds are also in custody, one of whom was found to have a sizeable cache of weapons when police apprehended him.

Police launched an investigation after receiving several reports of false bills circulating in Sweden last autumn.

“We have found money from the same source in circulation in society,” said public prosecutor Gunnar Fjaestad, who is leading the investigation, to DN.

Police are still unsure exactly where the bills may have been produced, however.

The suspected ring leader is a known figure to Scandinavian law enforcement, having been previously convicted of document forgery.

He was also one of four people arrested in Denmark several years ago for trying to sell a Rembrandt painting from Stockholm’s National Museum several years earlier.

The Supreme Court eventually had the 30-year-old and several other accomplices acquitted, citing methods used by the Swedish and American law enforcement to trap the man that were not allowed.