(S (S (BASENP (PRP It)) (VP (VP (VBZ is) (RB not)) (VP (VP (ADVP (RB yet)) (VBN known)) (SBAR (WHNP (WHNP (WDT which)) (PP (IN of) (BASENP (DT the) (NN two) (JJ Siberian) (NNS tigers)))) (S (VP (VBD was) (VP (VBN involved) (PP (IN in) (BASENP (DT the) (NNS attacks)))))))))) (PERIOD .))