You're thinking chemical. If biological, we wouldn't know until people started showing symptoms. But that's assuming the police wouldn't be testing for everything, which they probably will asap.
This is the type of thing I'm concerned about. An epidemic of pneumonic plague would send everyone into a panic and clog the health care system. It's contagious, and the symptoms are similar to inhalation anthrax. But it's treatable with antibiotics, so that's good.
Now smallpox is another thing entirely. But honestly, it's probably just amateurs, like someone else suggested.
I still think that it is horrible that, even if it was "amateurs" people will be relieved and, perhaps, feel this was insignificant. That makes a very sad statement about our world, and attitude of "gratefulness" when people can and do cause such chaos. We have become accustomed to expecting the worst, and being relieved when it does not turn out to be so terrible. It is terrible! Even the smallest of incidents is terrible. We cannot become complacent about any of these attempts.