The main suspects in episode 2, 'The Bitter Pill,' are James Lewis and his wife Leanne. James Lewis was the FBI's prime suspect in the Tylenol murders and was convicted of extortion for sending a letter to Johnson & Johnson taking credit for the crimes and demanding money, but he was never convicted of the murders themselves.

Stella Nickell is a key figure in the episode; she is suspected of poisoning her husband Bruce with cyanide-laced painkillers and then placing tainted capsules in stores hoping someone else would die, which led to the death of Sue Snow. Investigators found green crystals of algae destroyer, a fish tank product Stella had bought, mixed with the cyanide, linking her to the poisonings.

Investigators found little green crystals mixed with the cyanide in the capsules, which were identified as algae destroyer, a product used to kill algae in fish tanks. Stella Nickell had an aquarium and was confirmed to have purchased algae destroyer from a local fish store, providing a crucial clue tying her to the poisonings.

Bruce Nickell died suddenly, initially attributed to natural causes, but his wife Stella was suspected of poisoning him. She then allegedly tried to cover up by placing cyanide-laced capsules in stores, which led to Sue Snow's death after she bought a tainted bottle. The investigation linked these deaths through the cyanide poisonings and Stella's actions.

The Excedrin capsules were the source of cyanide poisoning that killed Sue Snow and Bruce Nickell. Three capsules in a bottle used by Snow were found laced with cyanide. The discovery led to a public recall of Excedrin products in the Seattle area and intensified the investigation into the poisonings.