In late June 1993, residents near Aum Shinrikyo's headquarters -- an utterly nondescript office building in an eastern area of Tokyo, and not, like, a giant skull-shaped island -- complained to local authorities about foul odors coming from the building. A few days later, the stink was accompanied by loud banging and a weird mist coming from the roof, and then that afternoon, by a black "gelatin-like" ooze creeping down the side of the building. Finally, on July 2, after the misting and ooze had continued unabated for at least two days, authorities politely asked the doomsday cult to vacate the building so they could investigate.
The cult complied, but investigators didn't get inside for another two weeks, at which point the building had been cleaned out, save for some black stains on the walls. The police took a few samples, but didn't bother to actually test them. And so, with little more than a series of halfhearted shrugs, authorities promptly forgot about the entire thing.
Fast-forward to 1995, when that cult tried to murder an entire subway system's worth of people. Someone involved in that investigation remembered the weird mist incident from earlier, and since they had a bunch of Aum Shinrikyo's goons and scientist-goons in custody, they finally asked them what was up. Anthrax was what was up. Aum Shinrikyo had successfully anthraxed Tokyo.
The only reason this didn't go down as a history-changing bioweapon attack is that the cult had accidentally released a strain of anthrax used for vaccines instead of a more murderous version. Reportedly, a few birds and pets died, but that apparently hadn't raised enough flags. But, again, neither had a doomsday cult's headquarters leaking black ooze, so what're you gonna do?
As for how many could have died, that's impossible to say. There were 7,000 people in the "high risk" zone, but who knows exactly how far a lethal strain would have spread. It would have come down to their spraying equipment, the viscosity of the compound, the weather/wind, and other factors we don't know because this exact thing has never happened before. For instance, if the first release had immediately worked, how many additional attacks could they have gotten in before somebody finally arrested them? Authorities seemed to have these guys on a pretty long leash.