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.

Pourandarjani was a doctor at Kahrizak, a prison on Tehran's outskirts where hundreds of opposition protesters were taken after being arrested in the crackdown following June's disputed presidential elections. The facility became so notorious that it was ordered shut down by Iran's supreme leader as reports of abuse and torture became an embarassment to the clerical rulers and security forces.

Pourandarjani later testified to a parliamentary committee and reportedly told them that one young protester he treated died from heavy torture.

The young physician died from an overdose of propranolol in a delivery salad, Dowlatabadi said Tuesday. Propranolol is used to treat high blood pressure, rapid heart rate and tremors, and can be lethal in high doses.

Investigators questioned the restaurant delivery man but he is not under arrest, Dowlatabadi said. The delivery man said he gave the salad directly to Pourandarjani, describing how the doctor took it from him at the door of his room, then closed the door behind him. The report did not say where the doctor was at the time.

Forensic tests showed that the doctor died of "poisoning by drugs" that matched the propranolol found in the salad, Dowlatabadi said. "A large number of these pills must be used for a person to pass away from them," he said.

Last week, Iran's top police commander, Ismail Ahmadi Moghaddam, insisted the death was a suicide, saying the doctor faced charges over failure to fulfil his duties to treat the detainees and killed himself in despair in a lounge at the courthouse. The police chief said a note was found with the body.

Moghaddam's comments, more than a week after the death, were the first public word that Pourandarjani faced any charges — or of where he died.

One pro-reform lawmaker dismissed the claims and suggested a link to the torture at the prison.

"It is impossible to accuse him of suicide," said Masood Pezeshkian, the pro-opposition Web site Roozonline reported Wednesday. "The idea of suicide by someone who had no problems and no serious disease — and was present during the events at Kahrizak — seems questionable to us."

The doctor's father, Reza-Qoli Pourandarjani, told The Associated Press last month that he didn't believe any of the causes given so far by the government in his son's death. But he didn't go as far as accusing anyone of killing him.

"Just the night before his death, my child talked to me on the phone, it was around 8 or 9 p.m. He sounded great, very dignified, displaying no sign of someone about to commit suicide," he said in a telephone interview from his home in Tabriz in northwestern Iran.

"He was even full of hope," and making plans with friends, the father said.

The next day, the elder Pourandarjani received a call from the commander of Tehran's security forces informing him that his son was in a car accident with a broken leg and needed his consent to have surgery. When he traveled to Tehran, "we found out that that wasn't the case," the father said.

Several opposition Web sites raised concerns that Pourandarjani was killed because he knew the conditions of a number of torture victims at Kahrizak, including 24-year-old Mohsen Rouhalamini, the son of a prominent conservative figure. Rouhalamini's death in late July was the main factor raising anger among government supporters over the abuse.

Hundreds of protesters and opposition activists were arrested in the crackdown that suppressed protests following the disputed June 12 presidential election. The opposition says at least 69 people were killed while the government has confirmed around 30 deaths.

More than 100 protesters, activists and pro-reform opposition have been on trial, accused of fueling the protests and being part of a plot to overthrow the government.