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After three murders with chillingly similar scenarios occurred in Austin, Texas, police began to wonder if they were chasing a serial killer. But whomever committed the crimes left no evidence, no DNA. Cops went into a frantic investigation. The hunt for clues changed when a man testing out a new thermal imaging scope recorded the nighttime movements of a stranger the night of the first murder . As "48 Hours" correspondent Maureen Maher reports, it was evidence cops never expected – and would help them break the case.