Max Pearson presents a collection of this week's Witness History and Sporting Witness episodes from the BBC World Service.

We hear about the unearthing of a mass grave in Sernyky, Ukraine, in 1990, and when the Boston Marathon was the target of a terror attack in 2013.

This programme contains distressing details.

Contributors:
James Bulgin - head of public history at the Imperial War Museum in Britain.
Richard Wright - archaeologist.
Jonathan Dimbleby - broadcaster.
Edward Deveau - Watertown Chief of Police.
Charles Barnett - managing director of Aintree Racecourse.
Gary Anderson - designer.

(Photo: David L. Ryan/The Boston Globe. Credit: Getty Images)