Man, this is a heinous act by a callous sociopath. In agreement with RALEIGH_COMP's beliefs, may these people be brought to justice. I've gotten first hand experience of such despicable people, and discovered how traumatic events in their past are the cause. BUT I can't condone or excuse their behavior and premeditated victimization of others.
I hope the police investigate. Spreading hundreds of tacks and brads means someone recently made a big purchase for this purpose. A plea for vendors to report any such purchases may narrow down who did this. If someone had wrecked from flats and gotten badly hurt, then this wouldn't be a lightly taken crime.
I wonder if they can run a Bobcat with an electromagnet to pick up the tacks and nails? Or at least a narrow road sweeper on the path to try to do the same.
The news article said the bridge was recently opened up to bicycles, whereas it had previously been a pedestrian-only bridge. I'll bet the culprit is a regular walker on the bridge who feels resentment about new bike traffic--perhaps someone who already has a history of berating cyclists there.