They could not recall that any of the passengers they screened were CAPPS selectees.
( They ( ( ( could not ) ( recall ( that ( ( any ( of ( ( the passengers ) ( they screened ) ) ) ) ( were ( CAPPS selectees ) ) ) ) ) ) . ) )
(ROOT (S (NP (PRP They)) (VP (MD could) (RB not) (VP (VB recall) (SBAR (IN that) (S (NP (NP (DT any)) (PP (IN of) (NP (NP (DT the) (NNS passengers)) (SBAR (S (NP (PRP they)) (VP (VBD screened))))))) (VP (VBD were) (NP (NNP CAPPS) (NNS selectees))))))) (. .)))
Perhaps some of the passengers they screened were CAPPS selectees, but they did not recall it. 
( ( ( ( ( Perhaps ( ( some ( of ( ( the passengers ) ( they screened ) ) ) ) ( were ( CAPPS selectees ) ) ) ) , ) but ) ( they ( ( did not ) ( recall it ) ) ) ) . )
(ROOT (S (S (ADVP (RB Perhaps)) (NP (NP (DT some)) (PP (IN of) (NP (NP (DT the) (NNS passengers)) (SBAR (S (NP (PRP they)) (VP (VBD screened))))))) (VP (VBD were) (NP (NNP CAPPS) (NNS selectees)))) (, ,) (CC but) (S (NP (PRP they)) (VP (VBD did) (RB not) (VP (VB recall) (NP (PRP it))))) (. .)))