YES this is what I suspected too, but..., such knuckleheads tend to "let their motives be known" even if it's by an anonymous tip. The fact that 150 years ago is 1871 and no slaves were enslaved at that time..., wouldn't make a difference to such "activists".
On the other hand it really could've been an accident as well as vandals using arson, just to watch the thing burn.
Anybody checked out if there is a large company that would benefit from a new bridge replacing the old covered one, that could handle larger, heavier trucks? Wouldn't be the first time a fire preceded "civic improvement"....
LD