From Northeast Intelligence Network:

27 July 2007: Ten people were treated for exposure to an unknown chemical substance after the as-yet unidentified substance was released in the Ann Arbor Public Library in the Westgate Shopping Plaza, and an adjacent grocery store Wednesday evening. According to a law enforcement source speaking to the Northeast Intelligence Network asking that his name not be used, “a person or persons unknown deliberately released a chemical into the air in two locations, one being the Ann Arbor Public Library and the second location being the Kroger Food Market.” According to this law enforcement source, the chemical appeared to be caustic as it caused breathing problems for patrons of the library and at the food store. The substance was initially released from the restroom of the library, possibly at the entrance to the restroom where a ventilation duct is located. This source declined to identify the manner in which the dispersal was made, adding that tests are being performed to identify the chemical.

Although law enforcement sources are stating publicly that they do not believe this incident is “terrorist related,” they deny knowing the motive of the incident…

Scary stuff. NEIN points out that this attack is similar to an earlier incident in a mall in Spokane, Washington.

It sounds like the chemical is mace or pepper spray to me, which would make it less a big “T” terror attack and more likely some sort of lefty political protest ala Andrew Stone, perhaps a statement against the Patriot Act’s infamous library reaserch clause and an attack on a corporate chain of groceries. Given the climate on the left, it’s fairly likely.
On the other hand it could be simple degeneracy, which is the story Ann Arbor authorities are sticking to:

ANN ARBOR — Police believe teenagers using mace or pepper spray at a local library and grocery store sickened about two dozen people.

Surveillance tapes showed the same two teens in the Ann Arbor library branch and adjacent Kroger store.

Ann Arbor police Sgt. Jeff Connelly said the incident appears to be a prank and that investigators had not identified the two males on the tapes, the Ann Arbor News reported Friday.

Emergency crews treated people at both locations shortly after 9 p.m. Wednesday for breathing troubles, burning eyes and dizziness. Five people were treated at the University of Michigan Hospital that night and released.

Tests on the chemical substance used were inclusive Thursday.

As a person who can sheepishly admit to accidentally macing myself (don’t ask) the burning eyes and breathing trouble sounds about right. But dizziness? Not so much, and certainly hospitalization would only be required of especially weak or frail persons. The EMT’s likely erred on the side of caution, but the fact that the police, who I suspect routinely pepper spray criminals, can’t identify for sure what these kids (if the kids sprayed anything at all, the footage doesn’t show them doing anything) supposedly sprayed in those buildings is troubling.