My thinking is more along the lines of the one witness, probably older, male etc having seen a person, perhaps OBG and another witness, perhaps young, or in other ways an unreliable sort having seen something else entirely. And that they then discounted the actual and accurate and important testimony. For example, if it’s true as I’ve read, that there are sometimes drug users under that bridge and one of them - perhaps high at the time, actually saw the victims being frogmarched down the hill and said what s/he saw - that could have been dismissed. In this theory there are two men, OBG and NBG, but because OBG was seen by an upstanding citizen and NBG by a marginalised member of society, the investigation pivoted. In the wrong direction.