[2] "The black echo" was a catchphrase developed by Bosch and several other soldiers in his unit, referring to the unique atmosphere of fear and uncertainty they experienced underground.

Based on his military history, Bosch suspects that Meadows' death may be connected to a spectacular bank robbery that occurred the previous month, in which the thieves employed the use of tunnels.

Because bank robbery is a federal crime, Bosch approaches the FBI's Los Angeles office and is introduced to Special Agent-In-Charge John Rourke and his subordinate, Agent Eleanor Wish.

The investigators also discover a connection with two Vietnamese expatriates who were high-ranking police officials in Saigon, then emigrated to the U.S. shortly before the end of the war.

Unfortunately, two internal affairs detectives who have been following Bosch mistake his surveillance for complicity in the robbery and insist on opening the vault, interrupting the thieves in the process.

Bosch fires at the thieves and follows them down into their tunnel but is wounded by their leader: Rourke, who as a military policeman in Vietnam worked with the government to convert the diamonds and assist with the emigration.

When her parents refused to tell her the truth, she investigated on her own and found that he returned to the U.S. on leave and had been killed when stumbled onto Rourke's scheme and tried to demand a cut.