But another aircraft was heading toward Washington, an aircraft about which NORAD had heard  United 93.
( But ( ( another aircraft ) ( ( was ( heading ( toward ( ( Washington , ) ( ( an aircraft ) ( ( about which ) ( NORAD ( had ( heard ( United 93 ) ) ) ) ) ) ) ) ) ) . ) ) )
(ROOT (S (CC But) (NP (DT another) (NN aircraft)) (VP (VBD was) (VP (VBG heading) (PP (IN toward) (NP (NP (NNP Washington)) (, ,) (NP (NP (DT an) (NN aircraft)) (SBAR (WHPP (IN about) (WHNP (WDT which))) (S (NP (NNP NORAD)) (VP (VBD had) (VP (VBN heard) (NP (NNP United) (CD 93))))))))))) (. .)))
Another commercial airliner was heading towards Washington State.
( ( Another ( commercial airliner ) ) ( ( was ( heading ( towards ( Washington State ) ) ) ) . ) )
(ROOT (S (NP (DT Another) (JJ commercial) (NN airliner)) (VP (VBD was) (VP (VBG heading) (PP (IN towards) (NP (NNP Washington) (NNP State))))) (. .)))