It sounds like something out of a children’s movie — after French police arrested three Polish thieves who attempted to steal toys in June 2020, specifically Lego sets. Now authorities are trying to address a growing trend of Lego robberies by warning business owners and parents, The Guardian reports.

According to a report in Le Parisien, the thieves were part of a gang that would stay in hotels near Paris, drive around in an Audi, and “raid” toy stores for valuable Lego sets to resell in Poland. The thieves were caught robbing a PicWicToys toy store, after being spotted in 2019 and 2020 at two separate European toy stores, La Grande Récré and Maxi Toys. French police are reportedly “building a case” against the gang in response, The Guardian writes.

The idea that plastic Lego bricks are worth enough to entice an international ring of thieves to steal them might seem odd — the priciest Lego set ever made retailed for only $800 — but some sets can be worth more over time. On top of that, Gerben van IJken, a Lego specialist who advises online collectible auction sites, tells The Guardian that interest in reselling Lego has grown dramatically in the last year:

Investing in these pieces isn’t new but this niche market has reached new heights with the pandemic. People have more time at home because of the health restrictions and the game market has exploded.

Van IJken cites one Lego set that retailed for €150 (around $212) in 2007 and resold for €2,500 (around $2,943) in 2020. That growth in value could be due to Lego’s practice of cycling sets out of production after around two years, van Ijken says. Keeping older sets “in the vault” (reminiscent of Disney’s strategy with home video) makes the ones still on the market or in owner’s hands that much more rare and valuable.

Lego thievery isn’t limited to France, either. A man reportedly stole around two million dollars worth of Lego sets in Florida in 2012. Given the risk, if you’ve got a collectible Lego space shuttle on the way, it might be time to think about security.