If this is CIA, then this is a bit of a sloppy job to kill these 3 people with the same symptoms within such a short time frame. They could have been slipped drugs by small doses or such that could cause heart attack, and I'm not sure if toxicology could catch these drugs. However, if I were the CIA, I wouldn't do it like that. These 3 people should die of different conditions (heart attack, stroke, seizure etc...) within some time of each other. This looks too suspicious to kill 3 people like that.
Rather, to have them all die within a short amount of time of the same symptoms suggests that they had something in common happen to them recently. Maybe they ate at the same banquet for engineers, or they had some activity together.
Also, it is well known that Yang Wei is China's most important engineer in the military aviation field. In addition, he oversees the J-20 program which is shaping up to be the biggest threat to American aerial dominance and it's in an advanced state of development as compare to the J-31.
If the CIA wanted to stunt Chinese growth, they would target Yang Wei first because killing 3 people in the same way within a month would put the Chinese on guard to defending its engineers, making killing Yang Wei much more difficult.
In any case, if I had authority, I would enforce a health check for all these important engineers to make some no one else was at risk of premature death and impose closer security around the food management and ventilation systems.