Aside from the fact that the J-Lo letter was inferred to be the source of Stevens' exposure by his co-workers and treating physicians, there are other interesting factors pointing to it as the source. One of the Delray Beach hijackers, Ahmed Alghamdi, reportedly subscribed to an AMI publication called Mira! devoted to Latin celebrities:
The Sun is a rather obscure target for an anthrax attack, whatever the source, but it is notable that the main hijacker nest in Delray Beach was rented to the hijackers by Gloria Irish, the wife of The Sun's editor, Michael Irish. Gloria Irish had multiple contacts with the hijackers over the months preceding 9/11. Michael Irish is a private pilot who flies out of the small Lantana airfield where Mohammed Atta brushed up his skills for knocking down the WTC.
The J-Lo letter was reportedly received on 9/4, seven days before 9/11. On that basis, if it had been opened immediately, people would have been coming down with anthrax at AMI just about the time of the attack.
Mohammed Atta had a sick sense of humor and apperantly enjoyed giving Americans a veiled heads up on what was coming -- see reports of his interview with the USDA loan manager. He also had a penchant for visual metaphors and riddles, viz:
"It was August 29 and my phone rang. It was Mohammed Atta. He said "I need your help with a puzzle". A puzzle? I said. Is this the right time to be talking about a puzzle? He replied "But only you can help me with this puzzle. What dooes 2 sticks, a dash and a cake and a stick mean? And he was talking about September 11."In addition to the lewd letter addressed to Jennifer Lopez, the "J-Lo" letter also apperantly contained a tobacco tin holding a Star of David buried in powder, a cheap cigar in its cylinder, and a laundry detergent package (presumably of the small, coin-op dispenser type). The brand of the latter is unknown, but if it looked like this, it would make a certain amount of sense:
All in all (no pun intended) I don't think the publicly-available evidence is definitive. But for myself, I regard the J-Lo letter as guilty until proven innocent.