Speaking to Aljazeera from Damascus on Sunday, the head of the Arab Organisation for Human Rights in Syria, Muhammad Raadoun, said that Syrian dissident Ahmad al-Masalma had died due to a denial of basic health care.
"The authorities considered his death fate but in fact by preventing al-Masalma from receiving the required medical care at the proper time they caused his death," Raadoun said.
Al-Masalma was arrested on returning from 25 years' exile in January 2005 and died in April.
An ex-member of the banned Muslim Brotherhood movement, the 65-year-old had returned to Damascus after receiving assurances he would be safe to do so.
His supporters said they had warned the authorities that al-Masalma needed special care because of heart problems. He was released to undergo a heart operation but died in hospital.
Al-Masalma left Syria in the early 1980s following fierce clashes between members of the Muslim Brotherhood and the Syrian army in the northern province of Hama in which as many many thousands of people were killed.
He was living in Saudi Arabia before his return earlier this year. A number of Syrian exiles have recently returned including a military officer who was sentenced to death after a failed coup attempt more than 40 years ago.
The Arab Organisation for Human Rights in Syria has demanded that the authorities conduct a thorough investigation into the circumstances of the death.