[theDentist]

... he opened a suspicious-looking letter with a California postmark

Was it suspicious looking before he opened it, or after he poured out the powder. Seems to me if it were "suspicious", he should have contacted police or fed or whoever to open it under proper conditions.

[dead]

Was it suspicious looking before he opened it, or after he poured out the powder.

It became suspicious right after he poured in the powder.