A doctor who exposed the torture of jailed protesters in Iran died of poisoning from an overdose of a blood pressure drug in a salad, prosecutors say. The findings fueled opposition fears that he was killed because of what he knew.

Investigators are still trying to determine whether his death was a suicide or murder, Tehran's public prosecutor Abbas Dowlatabadi said, according to the state news agency IRNA.

The 26-year-old doctor, Ramin Pourandarjani, died on Nov. 10 in mysterious circumstances - with authorities initially saying he was in a car accident, had a heart attack or committed suicide.