The Nightriders

From back of book:

With rumors swirling around a series of stolen gold shipments, Wells Fargo sends Zachary Cobb to California gold fields to investigate the unsolved robberies. A gang of vigilantes called the Vindicators have taken it upon themselves to prosecute men suspected of the hold-ups, but while innocent people are being lynched, the real bandits seem able to strike with impunity any time they choose.

Maintaining his undercover status by visiting a cantankerous aunt who lives nearby, Zac discovers that the Vindicators are controlled by corrupt federal officials. He also discovers the judge to be a former commander of a Union prison camp where two of his bothers were imprisoned during the Civil War.

Emily Morgan, whose fiance was lynched by the vigilantes and whose brother is the local sheriff, helps Zac in his search. She is determined to bring all those involved to justice, even if it includes her own kin.

But when Jenny Hays shows up at his aunt's house and is pulled into the mystery, Zac has much more than a dangerous investigation to solve!