The Witness Must Die by Alex R Carver

Sofia Torres is the only witness when her employer and his entire household are massacred by a rival druglord.

The Spanish police are relieved to have a witness, but they soon discover that keeping her alive to testify won't be an easy job, especially when the man responsible for the massacre is prepared to kill as many people as it takes to keep her quiet. Even with the help of Interpol, keeping Sofia Torres safe is difficult and costly, and with the eyes of the world on Barcelona as it becomes like Chicago in the 1920s they must rely on help from an unexpected source.