The Fallen by David Baldacci

Amos Decker works with the FBI. A former cop, he has perfect memory of everything he's ever encountered, thanks to a blow to the head. Amos and his partner Alex go on vacation to her sister's home in rundown, drug-infested Baronville, where Alex's brother-in-law is the recently hired manager at a big shipping distribution facility. After dinner one night, Amos is outside and notices lights flickering in the empty house next door. When he goes to investigate, he finds 2 dead men. His attempts to identify the 2 men lead to the arrival of the DEA on the scene. And the opening of a big can of worms. Amos and Alex almost get killed trying to find out who is behind those 2 murders as well as a string of other probably-related  murders. It's quite the page-turner.