HCl (chlorine stripping hydrogen from water) causes burns and hemorrhaging of mucosa and lung tissue. Persons exposed get a whiff and run if that option is available. Enough chlorine to quickly incapacitate persons inside a structure would linger and be evident to responders.
If chlorine gas was the culprit, it could explain the additional injuries to the 4 first responders.
Then again it wouldn’t be too ‘mysterious’ to deduce the source.