Complex sentence: This figure includes five riflemen who had been captured but were subsequently shot by their guards; according to Marie, the guards believed the British were attempting to escape when they spontaneously took cover in a ditch 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.