About 20 hackers told the BBC they were stealing the private gaming accounts of players and reselling them online.
Fortnite is free to play but is estimated to have made more than £1bn through the sale of "skins", which change the look of a character, and other add-ons.
This fuels a growing black market.
Hackers can sell player accounts for as little as 25p or hundreds of pounds, depending on what they contain.
The items are collected as in-game purchases but are purely cosmetic and do not give gamers any extra abilities.
Fortnite-maker Epic declined to comment on the investigation but said it was working to improve account security.
The game has more than 200 million players.