The Shadow Murders — Jussi Adler-Olsen

In the penultimate thriller of the New York Times and #1 internationally bestselling Department Q series, the team must hunt for a nefarious serial killer who has slipped under the radar for decades.

On her sixtieth birthday, a woman commits suicide. When the case lands on Detective Carl Mørck's desk, he can't imagine what this has to do with Department Q, Copenhagen's cold cases division. It's a tragedy to be sure, but the cause of death seems to be clear. But his superior, Marcus Jacobsen, is convinced that this is not in fact a suicide, but a murder related to an unsolved case that has been plaguing him since 1988.
 
At Marcus's behest, Carl and the Department Q gang -- Rose, Assad, and Gordon -- reluctantly begin to investigate. However, they quickly discover that Marcus is on to something: Every two years for the past three decades, there have been unusual, impeccably timed deaths with connections between them that cannot be ignored. As they dig deeper, it becomes clear that these “accidents” are in fact murders by a very cunning and violent serial killer.
 
Faced with their toughest case yet, made only more difficult by COVID-19 restrictions and the challenges of their own personal lives, the Department Q team must race to find the culprit before the next murder is committed, as it is becoming increasingly clear that the killer is far from finished.