Star Trek: Mission Gamma, book 04 : Lesser Evil
Robert Simpson2002

Chaos erupts aboard Deep Space 9 as the crew begins a desperate search for a killer in their midst, catapulting Colonel Kira Nerys on a dangerous chase into the heart of the Federation. But the crime she seeks to avenge is part of a plot more ancient and far-reaching than anyone suspects, and that secret, if exposed, could divide worlds throughout the Alpha Quadrant.

Meanwhile, as the Starship Defiant makes its way back toward the wormhole and home to DS9, a startling discovery shakes Commander Elias Vaughn to his core and brings to light the truth behind the most tragic mission of his long life. As the crew struggles with the implications of what they've found, their captain's judgment comes into question...and casts doubt on the final outcome of their historic voyage through the Gamma Quadrant.