I would add to Karl's excellent list of American compositions suitable
for memorializing 9/11:

John Williams: Hymn to the Fallen  (a stunning concert work, adapted
               from Mr. Williams's score to Saving Private Ryan)
Stephen Shewan: Elegy, God Shall Wipe Away All Tears
Samuel Barber: Agnus Dei (choral version of the Adagio)
Roy Harris: Symphony No. 6
Morten Lauridsen: Lux Aeterna
William Schuman: Carols of Death

And to the "international" list:

Edward Elgar: Nimrod, from the Enigma Variations
George Lloyd: Sanctus, from the Symphonic Mass
Benjamin Britten: War Requiem
Hector Berlioz: Symphonie funebre et triumphale

One could name more....

John Proffitt
KUHF-FM Houston