Payday: The Heist developer Overkill Entertainment is working on a prequel to Valve's Left 4 Dead, a reliable source has told CVG.
The PSN dev announced a partnership with Valve late last month, although it wasn't immediately clear what form its collaboration would take.
We're now told it's a project that will tell the story of the zombie outbreak that happened before the original Left 4 Dead games.
In its vague announcement Overkill described the project as a "very cool blend of Payday and Left 4 Dead".
"As perceptive gamers will have noticed, several hints have recently been dropped into Payday: The Heist, which has led to various rumors," a statement read. "We are excited to be able to confirm that an in-depth collaboration between Overkill and Valve is currently in production.
"We are working on a very cool blend of Payday and Left 4 Dead. I am sure it is so exciting that it will have some players check into the hospital before we are done."
Overkill was recently acquired by The Chronicles of Riddick: Escape from Butcher Bay, The Darkness and Syndicate maker Starbreeze.
A sequel to Payday: The Heist is also currently in the works.