End Game by Tracey Devlyn

An FBI agent assigned to investigate a meddlesome lobbyist for her role in a possible pay-to-play scheme discovers a secret political organization willing to destroy everything to attain ultimate power—including the woman he loves.
In this highly anticipated, fifth and final installment of the Steele Ridge: The Blackwells series, USA Today bestselling author Tracey Devlyn thrusts readers into the murky world of high-stakes politics and complex family relationships, wrapped in a sizzling enemies-to-lovers story.
As a savvy and successful lobbyist, Kayla Krowne knows all about deception. Each day, she swims in a vast tank of opportunistic sharks. But nothing prepares her for the downward spiral her life takes after witnessing the assassination of a high-profile public official. When the police suggest she may have been the true target, her only hope of rooting out the murderer lies with the one Blackwell brother she’s never been able to win over. The one she’s held a secret longing for since their first meeting two years earlier.
Special Agent Asher Cameron Blackwell’s day started off bad and ended worse. Not only is he drowning in a swamp of paperwork, he now has two investigations involving none other than Kayla-effing-Krowne. One is a presumed assassination case and the other one he’s compelled to keep from her because of a promise he made to his semi-estranged brother, Zeke.
Forced into close proximity, Ash and Kayla finally give in to the passion they can no longer hide, forging an unstoppable union as they navigate an intricate maze of lies and betrayals, and uncovering a heartbreaking conspiracy that leads them to Kayla’s doorstep. It’s a life-and-death battle that they dare not lose.