Investigators later found the spot where the killer had waited to take aim, the committee said -- a stairway between the fifth and sixth floors of a nearby building.

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That decision involved taking Halliwell to a beauty spot at Barbury Castle near Savernake Forest, where the detective "appealed to the killer's conscience" over Sian.

BBC: Sian O'Callaghan's body found after killer's admission

For a former officer to be a suspected cop killer is beyond imagination for many law enforcement officers, whose culture bonds them so tightly that many can spot another officer out of uniform just by their demeanor and a nod of the head.

CNN: STORY HIGHLIGHTS