(S (S (S (BASENP (PRP They)) (VP (VBD found) (S (BASENP (DT the) (NN tiger)) (VP (VBG attacking) (NP (BASENP (NN one)) (PP (IN of) (BASENP (DT the) (NN zoo) (NNS visitors)))))))) (SBAR (WHADVP (WRB when)) (S (BASENP (PRP they)) (VP (VBD killed) (BASENP (PRP it)))))) (PERIOD .))