Unfortunately, "what suicide bombers were" an excerpt from the news article. In my opinion, there is no difference following the sentence. I know who he is. I know what he is
I know what suicide bombers are; they are people whose are prepared to sacrifice their lives for their cause. I know who suicide bombers are. They are members of the X organisation.
In practice there may not be a great deal of difference between 'who and 'what' those two sentences. There is some difference in the following two: