Remedy is best known for the Max Payne series and the recent TV show/game experience that is Quantum Break, all of which are single-player only games. That will change with the next game.
In a news post on their site, the Finnish developer stated that they are changing things up a bit. While acknowledging that they have largely focused on creating "exciting stories, memorable characters, believable worlds and cinematic action" within a single-player context, they want to try to bring all of that into a co-op experience.
Single-player experience has always been an important focus for us. At the same time, we want to challenge ourselves to find ways to also expand these elements to include cooperative multiplayer.
We will always strive to be the best storytellers we possibly can, and we want these stories to be shared and elevated through players’ cooperation. We believe that a multiplayer experience combined with Remedy’s unique world-building provides an exceptional environment for that.
- Mikael Kasurinen (Game Director) & Sam Lake (Creative Director)
The post concludes with Remedy stating they they are hiring a Lead Network Programmer to help them shape "the multiplayer technology and features of [Remedy's proprietary game engine] Northlight." So while we don't have a lot of other info at this point, it sounds pretty certain that the mystery game will support online co-op.