The authorities are now looking for anything that might have been part of the explosive devices - bits of metal or sharp objects that they might be able to trace back to its owner or owners.

BBC: Forensics investigators decode the Boston bombings

The team's work offers such a sharp picture of the past that it is possible to trace the histories even of individual communities, such as one in Essex whose inhabitants built, used and then abandoned an enclosure within the span of a single generation.

ECONOMIST: The Neolithic

Manfred Kayser says ancestry markers can - at best - trace an individual to a broad region: "Human migration history has produced no sharp genetic borders and no sharp appearance borders, " he explains.

BBC: Painting a suspect's portrait with DNA