What key piece of testimony lead authorities to convict Cathy Smith of manslaughter in the death of John Belushi

Cathy Smith injected heroin in the body of John Belushi.

Explanation:

In the year 1986, Cathy Smith surrendered herself to the police accepting her crime saying that she was responsible for the death of Belushi as she injected huge amount of heroin in the Belushi and due to overdose of the drug he died. This incident took place in the year 1982 . After that Cathy Smith was imprisoned for his crime for 15 months. Now after so many years Cathy Smith is suffering due to poverty and she is not allowed to perform in many local bars.