Reports by the CIA confirm that it created dart guns in the 1970s to kill targets by causing paralysis and not by heart-attack.
The Washington Post's article archived by the CIA library states that the top officials of the CIA had discovered the deadly shellfish toxin which was used in their secret dart gun, capable of killing without the targets realising that they had been hit. The shellfish toxin-based dart would attack the central nervous system causing paralysis in the human body (Refer to Page No.2).
The Paralytic Shellfish Poisoning (PSP) fact Sheet released by the Alaska Department of Health and Social Services says that the toxin levels within a single shellfish could be fatal to humans. It adds that death could occur in as little as 2 hours from paralysis of the breathing muscles if exposed to high levels of the toxin.
As per the cited reports, shellfish toxin released into the target would have been almost undetectable in the body, suggesting that the death might have been ruled out as a heart attack.