At 8:21, American 11 turned off its transponder, immediately degrading the information available about the aircraft.
( ( At 8:21 ) ( , ( ( American 11 ) ( ( ( ( ( turned off ) ( its transponder ) ) , ) ( immediately ( degrading ( ( the information ) ( available ( about ( the aircraft ) ) ) ) ) ) ) . ) ) ) )
(ROOT (S (PP (IN At) (NP (CD 8:21))) (, ,) (NP (NNP American) (CD 11)) (VP (VBD turned) (PRT (RP off)) (NP (PRP$ its) (NN transponder)) (, ,) (S (ADVP (RB immediately)) (VP (VBG degrading) (S (NP (DT the) (NN information)) (ADJP (JJ available) (PP (IN about) (NP (DT the) (NN aircraft)))))))) (. .)))
American 11 turned off its transponder at 8:21 AM immediately degrading any available information about the location or wellbeing of the aircraft which may have been done by the terrorists in an effort to make it harder for the military to track the airplane. 
( ( American 11 ) ( ( ( ( turned off ) ( its transponder ) ) ( at ( ( 8:21 AM ) ( ( ( immediately ( degrading ( ( any ( available information ) ) ( about ( the location ) ) ) ) ) or ) ( wellbeing ( of ( ( the aircraft ) ( which ( may ( have ( been ( ( done ( by ( ( the terrorists ) ( in ( an effort ) ) ) ) ) ( to ( make ( it ( harder ( for ( the ( military ( to ( track ( the airplane ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (NNP American) (CD 11)) (VP (VBD turned) (PRT (RP off)) (NP (PRP$ its) (NN transponder)) (PP (IN at) (S (NP (NNP 8:21) (VBP AM)) (VP (VP (ADVP (RB immediately)) (VBG degrading) (NP (NP (DT any) (JJ available) (NN information)) (PP (IN about) (NP (DT the) (NN location))))) (CC or) (VP (VBG wellbeing) (PP (IN of) (NP (NP (DT the) (NN aircraft)) (SBAR (WHNP (WDT which)) (S (VP (MD may) (VP (VB have) (VP (VBN been) (VP (VBN done) (PP (IN by) (NP (NP (DT the) (NNS terrorists)) (PP (IN in) (NP (DT an) (NN effort))))) (S (VP (TO to) (VP (VB make) (S (NP (PRP it)) (ADJP (JJR harder) (PP (IN for) (NP (DT the) (NN military) (S (VP (TO to) (VP (VB track) (NP (DT the) (NN airplane))))))))))))))))))))))))) (. .)))