The Children of Eve by John Connolly

Wyatt Riggins, the boyfriend of rising Maine artist Zetta Nadeau, has gone missing, leaving behind a cell phone containing a single-word message: RUN. Private investigator Charlie Parker is hired to find out why Riggins has fled, and from whom.
Parker discovers that Riggins, an ex-soldier, has been involved in the abduction of four children from Mexico: three girls and a boy, all belonging to the cartel boss Blas Urrea—except Urrea’s family is safe and well in Mexico, which means the abductees cannot be his children. Yet whoever they are, Urrea wants them back, and has dispatched his agents to secure them, even if it means butchering everyone who stands in their way.

One of those agents is Eugene Seeley, a clever, ruthless solver of other men’s problems. The other is an unknown woman.
Every child has a mother. Now Charlie Parker will face one unlike any other, and learn the terrifying truth about the Children of Eve.