(S (S (BASENP (DT The) (NNS authors)) (VP (ADVP (RB also)) (VP (VBD concluded) (SBAR (SBAR (IN that) (S (BASENP (JJ military) (NN air) (NN activity)) (VP (VP (VBD was) (ADJP (JJ heavy))) (PP (IN at) (BASENP (DT the) (NN time)))))) (CONJP (COMMA ,) (CC but)) (SBAR (IN that) (S (BASENP (DT the) (NN radar) (NNS records)) (VP (VBP show) (NP (BASENP (DT no) (JJ overt) (NN action)) (PP (IN toward) (BASENP (DT the) (JJ unknown) (NN object))))))))))) (PERIOD .))