(S (S (S (BASENP (DT All) (NNS hijackers)) (VP (VBD were) (VP (VBN arrested)))) (SBAR (IN after) (S (BASENP (JJ Mexican) (NN security) (NNS forces)) (VP (VBD boarded) (BASENP (DT the) (NN plane)))))) (PERIOD .))