I agree with the person who says that it was a poorly constructed sentence.
I believe Obama meant to say: 'We're heartbroken that something like this has happened again'. He, justifiably, went for the subjunctive but got it wrong. What he should have said was:
'We're heartbroken that something like this should have happened again'.
Edit After reading what others have written I am less sure of my earlier expressed opinion on this.
One thing is certain and that is that Obama's sentence is grammatically incorrect. But I think there are two possibilities as to what he meant to say:
Either a) he was trying to use the subjunctive and got it wrong (see my above comments); or
b) he was saying he was heartbroken that something like the previous atrocity 'might' have occurred again. But if that was the case surely he should have said 'We are heartbroken that it might be a case of another terrorist atrocity'. But he really ought to be 'heartbroken' whatever the cause, since someone has died. All in all it is a garbled statement, but perhaps understandable in the circumstances.