In Sequence by Cole McCade

The Golden Ratio killer resurfaces in California to target a former victim, and this time the case takes Malcolm and Seong-Jae hunting deep into his past, searching for connections between victim and killer that may reveal his identity. But with the killer one step ahead of them, toying with them, leaving false trails at every turn, it’s like playing Russian Roulette — and if they guess the wrong suspect out of a potential five, one misstep could leave many more dead.
With Malcolm and Seong-Jae’s domestic life — and sex life — up in the air they’re off their game, off the mark, and already starting to crack. But they aren’t the only ones, as back in Baltimore Huang kicks a new plan into motion and Anjulie finds herself under unwanted scrutiny. Everyone’s lives are falling apart.
And if they can’t get their sh*t together, more than their relationships may crash and burn.