[Nogbad]

The hijacker, Ahmed Ibrahim A. Al Haznawi,
was examined days after he entered the United States,
an indication that the infection
— whatever it was —
developed before his arrival.

And the letters were xeroxed on paper
of a size rarely found in the USA
but common in Europe.

(Remember that?
It was in a WSJ article posted here).

Hmmm.

[Mitchell]

And the letters were xeroxed on paper of a size rarely found in the USA but common in Europe.

As I recall, they were A4 size, or possibly A3 (which I think is drawing paper, twice as wide as A4 but the same height), cut in two or trimmed. This is from memory; I don't have a reference right now.

[Revelation 911]

(Remember that? It was in a WSJ article posted here).

....also posted here - mad scientist, aryan hate group, "conservative" extremists etc stc. (sarcasm) - I agree with you though, this seems the most plausible. We just need to ask ourselves why the others were put forth - any ideas