That's their main reason for pointing at him.
But he also worked at USAMRIID ten years before the anthrax attacks of 2001. (At the time of the attacks he lived in Colorado.)
He was also seen entering the facility without permission back then.
He's also a Veterinarian with knowledge of anthrax.
There was also a letter sent after 9/11 but before the anthrax attacks became known which told the FBI to investigate an Arab-American who once worked for USAMRIID. That Arab-American seems to think that Zack (or someone like Zack) was behind the letter. And because of the timing of the letter, he thinks the letter may have been sent by the anthrax mailer.
So, there are other reasons besides the main reason. But those other reasons don't add up to much. It's all in the article.
I have always gotten the impression as well that it was primarily the Pat Buchanan type crowd that suspected Zack the most. The fact that he’s Catholic makes it almost amusing!