The always good NY Times science page has a nice article by Gina Kolata about the genetic reconstruction of the 1918 flu . Pieces of that virus were amplified from its 1918 victims, and overall the virus quite clearly belongs in the avian influenza group.
The problem is, the details of the sequence look different from any of the known flu variants- different from strains found in american fowl; different from the H5N1, the current bird flu; and different even from avian flu obtained from preserved animals from 1918. Specifically, the hemagluttinin gene, which is necessary for the 1918 virus' huge virulence, has about 30 amino acid substitutions relative to known avian strains. These changes make a new structure for hemagluttinin which lets it infect mammalian cells.
The scientists involved are now examining migrating bird populations to isolate a closer relative of the 1918 monster flu.