Agreed, I think the only two possible answers are (a), ingestion; (b), inhalation. You might be able to hide something like a needle mark; but I doubt you could inject multiple people against their will without them putting up a fight, so injection's out, IMO.
The only issue I see with the inhalation theory is this one quote from Police Chief Ziegler:
“These four people didn’t do it themselves,” Ziegler said, noting it’s quite a large crime scene and no suspects have been identified.
Four bodies found at RJR Maintenance and Management
The offices are up front, and there is a warehouse in the back, so wondering how large a crime scene it could be.