In (an) island x on (an) island: Qual a forma correta?

Hi folks!

So here's my doubt:

Yesterday I was reading some news on Cnn.com and I saw this: "...people were killed on Utoya island, in the deadliest..." Now the question: Why "on Utoya"? And why not in Utoya?
Because as far as I know, before places the preposition is in!

If you want to check the news: http://edition.cnn.com/2011/11/14/world ... =allsearch

Thanks in advance =^.^=