Sharon, that's terrible. I'm so sorry for your loss, and your neighbour's.

Has a vet confirmed that both cats were poisoned, and what with? If so, I guess it may not have been deliberate (though it may have been), but maybe somebody in a nearby garden has been putting out poison to destroy rats or mice, and passing cats have been eating it, or eating the poisoned rodents. Or leaving chemicals around.

I'd be happy to join some kind of campaign to try to trace the source of the problem and sort it out, but I'm not sure how best to start.