I assume there will be a very, very careful analysis of the captain's blood chemistry, along with his water, food and beverages. Although his choice of words leads us down other routes, a senior man's sudden departure from a well-balanced state might well have reasons other than natural causes.
Clearly, I'm suggesting that all crews have a vulnerability to being intentionally incapacitated with drugs - perhaps even sophisticated and difficult to detect chemistry used in research.
I would also test for connections with other cases where data and samples are still available. Nothing is confined to the logic of fiction these days.