Here's a couple from 2012:

http://deadspin.com/5896709/racist-tweets-about-fabrice-muam...

http://www.theguardian.com/technology/2012/mar/21/man-racial...

Here's 11 people who got locked up last year:

http://www.dailymail.co.uk/news/article-2330809/Lee-Rigby-de... (daily fail, but their info appears sound)

And that was just on the first page of google. Note that in all of these cases, the comments were just ugly and racist, but not threatening.