Many Americans were touched by the outpouring of sympathy by folks around the world after 9/11. Those who want to return the favor and send a card or flowers to the Spanish embassy in Washington, DC, the address is:
2375 Pennsylvania Ave. N.W.
Washington, DC 20037
If sending flowers, you may need the phone number: (202) 452-0100.
Thanks for the info.
There are also several other consulates (I almost said consolates, which is sadly appropriate) across the US, listed here. Many of them have set up guest books you can sign to leave your condolences, should you choose.