(S (S (BASENP (NNS Police) (NNS efforts)) (VP (VBP are) (ADJP (JJ underway) (S (VP (TO to) (VP (VB track) (NP (BASENP (DT the) (NN origin)) (PP (IN of) (NP (BASENP (DT the) (NN letter)) (SBAR (WHNP (WDT that)) (S (VP (VBD initiated) (BASENP (DT the) (NN scare)))))))))))))) (PERIOD .))