Winter's Ghost by Mary Stone

 prisoner responsible for a mall massacre that occurred the night The Preacher was taken down is executed, with a well-placed bullet fired from nearly a mile away. The sniper is clearly a professional—either military or law enforcement—and leaves zero evidence, other than a note.

When a series of suspected rapists and murderers turn up dead, the killer’s pattern becomes clear: they’re acting as judge, jury, and executioner for cases that were brushed off by the cops. How could a person not cheer a little?

Until the spotlight is shone on one of the FBI’s own.

Ultimately, it’s a matter of right or wrong. Winter knows just where that line is—she learned the night her parents were slaughtered and her baby brother disappeared. That night made her who she is, and she’ll uphold the law, even for scumbags who deserve to die. Even while the ghosts of her past grow closer and closer.