If I recall, there were conflicting "Martial law" orders and reports floating around. If I'm a sherrif and the last word I had was that martial law had been declared, then no one is getting across that bridge in any way shape or form, unless they're on a stretcher.
It has never been considered here, I think, that perhaps the situation was so fluid that the sherrif made a decision based on the last (and inadequate) information he had; that martial law had been declared and the images of chaos, looting and random gunfire that CNN enjoyed showing so much.
Did anyone ever bother to find out if the man had adequate communications to/from his higher ups or state authorities?
P.S. I'm sure that in a city surrounded on three sides by water, all roads and bridges are emergency evacuation routes. Was this the only one that was closed? I doubt it.
If martial law is declared, then a local law enforcement officer no longer has any power - martial law means that the military assumes control of law enforcement.
If you're a sheriff and the last word you had was that martial law had been declared, you're no longer in charge of diddly. The military is.