"Following the attack lifeguards urged people to stay out of the water, but temperatures were so high that after half an hour bathers returned to the river."
This doesn't sound right, if it was a true feeding frenzy the worst of the injuries would not have been a missing pinky and a toe.
This sounds like a group of people got too close to their nest and they were just defending the breeding ground. They tend to breed in higher temps which makes perfect sense. If an entire shoal of these fish really wanted to do damage it would be alot worse and get national TV coverage.
That said it doesn't even fit their characteristic, I've seen people with open wounds put their hands in a rather large piranha tank and the fish ignore it all together, they're pretty good at know what's dinner and what's not really worth it.