Complex sentence: This figure includes five riflemen, that had been taken prisoner, who were shot by their guards who believed they were attempting to escape when they dived into a ditch for cover as American shellfire closed in.
Split into simpler sentences: This figure includes five riflemen who had been taken prisoner and were subsequently shot by their guards. It is understood that the guards themselves believed the British were attempting to escape when they dived into a ditch for cover as American shellfire closed in.