Many of those held at the Kahrizak facility were opposition supporters
Iran's police chief says a doctor who died at a detention centre shut in July after the deaths of several opposition supporters committed suicide.
Gen Esmaeil Ahmadi-Moqaddam said a note had been found next to the body of Dr Ramin Pourandarjani last week, which suggested he was feeling depressed.
Opposition groups and some MPs have described his death as "suspicious".
Dr Pourandarjani had reportedly given evidence to a parliamentary committee about abuse at the Kahrizak centre.
Opposition websites say he had admitted to being forced to say that one detainee had died of meningitis, and to say nothing about what he had seen.
Many of those held at Kahrizak were people who had protested against the re-election in June of President Mahmoud Ahmadinejad.
At least 30 protesters have been killed in clashes since the poll. Thousands were detained and some 200 activists remain behind bars. At least five people have been sentenced to death, prosecutors say.
Iran's Supreme Leader, Ayatollah Ali Khamenei, ordered Kahrizak to be closed because it had failed to "preserve the rights of detainees".
Officials said two opposition activists had died at the centre after contracting meningitis, but opposition websites alleged that many more had died after being tortured and abused by the authorities.
Dr Pourandarjani, a general practitioner, had been working there once a week as part of his military service and is believed to have treated several detainees following June's election unrest.
After the 26-year-old's death was reported last week, officials initially said he had suffered a heart attack in his sleep. They then claimed that he had died of poisoning, before saying on Wednesday that he had committed suicide.
Gen Ahmadi-Moqaddam said Dr Pourandarjani was depressed because he was facing trial and had written "a will" in which he "complained about warnings he had received of a five years' jail term if convicted", according to the ILNA news agency.
He said the coroner would shortly rule on the cause of death.
But Dr Pourandarjani's father said he spoken to his son on the eve of his death and that he had showed no signs of depression.
"He sounded great, very dignified, displaying no sign of someone about to commit suicide," Reza-Qoli Pourandarjani told the Associated Press.
Opposition websites have alleged that Dr Pourandarjani was killed because he knew what had happened to a number of detainees who died at Kahrizak, including Mohsen Rouhalamini, the son of a prominent conservative.
The Mowjcamp website said he had told a parliamentary committee investigating abuse that Rouhalamini had been brought to him "in a dreadful state after being subjected to extreme physical torture".
After the 24-year-old died, "officials in Kahrizak threatened that if I disclosed the causes of the wounds of the injured at Kahrizak, I would not be able to live," the website quoted the doctor as saying.
Dr Pourandarjani was then detained himself until he agreed to state that the cause of Rouhalamini's death was meningitis, it said.