Because the more they investigated, the more it looked like the agents fell into a culvert in the dark. In fact, the emergency dispatcher who took the surviving agent's radio call wrote in the log, "(He) thinks they (both agents) ran into a culvert," according to the FBI report.
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That is what the FBI claimed at the time. In all honesty, can the FBI be trusted?
A year later, they admit, they don't know what happened. A year later: Agent Martinez's death still unsolved; reward increased to $50K
So you think that the FBI is lying when they say that’s what the dispatcher wrote in the log?