Whodunit?

In the end, it was a slice of pizza that sealed his fate: In a scene straight from CSI, a police officer disguised as a waiter retrieved the partly eaten crust and tableware abandoned by the suspect on a restaurant table. Soon, tests confirmed that the DNA in saliva on the pizza matched evidence from a crime scene. That suspect, Lonnie Franklin Jr., is accused of being the serial killer nicknamed the “Grim Sleeper,” who left a trail of at least 10 victims in Los Angeles over a 25-year span. He was arrested in July and has pleaded not guilty to murder charges.

Italian food may have been Franklin’s downfall, but a less common, and more controversial, forensic technique led police to the suspect in the first place. Known as familial DNA searching, it scours existing DNA databases for partial matches that suggest an unidentified suspect may be a close relative of someone in the database. In the case of the Grim Sleeper, such a search pinpointed DNA that partially matched DNA found at one of the killer’s crime scenes...