(S (S (BASENP (PRP They)) (VP (VBP express) (NP (BASENP (NN concern)) (PP (IN about) (NP (BASENP (DT the) (NN possibilty)) (SBAR (IN that) (S (BASENP (DT this)) (VP (MD could) (VP (VB have) (VP (VBN been) (NP (BASENP (DT a) (NN terrorist) (NN aircraft)) (PP (IN with) (BASENP (DT no) (NN transponder)))))))))))))) (PERIOD .))