I've done some research and it appears that it would be pretty difficult to get an actual trending topic on Twitter. Part of the problem is that you need a lot of different people tweeting about the topic. It's not enough to have a few people tweeting repeatedly.
There is still a lot that we can do by tweeting and retweeting these news stories. One day we might get lucky and have a news item really take off. It's already happening with this latest case. It just hasn't been enough to trend yet.