Thet Win Aung, who was sentenced to 59 years’ imprisonment after protesting for educational reform, has died in a central Myanmar jail, human rights groups said Tuesday.

London-based rights group Amnesty International called on Myanmar authorities to launch an independent investigation into his death Monday, while another human rights group said it believed the ailing prisoner had been “systematically killed by the military regime.” There was no immediate comment available from the Myanmar government, which is said by human rights groups and the United Nations to hold more than 1,100 political prisoners under inadequate and brutal conditions.

Thet Win Aung, 34, had been in prison since 1998 for organizing small, peaceful demonstrations calling for improvements in the educational system and the release of political prisoners. Ten years earlier, he took part in the pro-democracy uprising led by detained Nobel Peace Prize laureate Aung San Suu Kyi.

Amnesty International said he was badly tortured during his imprisonment and suffered from a variety of health problems, including malaria. By 2005 he was reported to have been unable to walk without assistance.

The Assistance Association of Political Prisoners, a group of ex-political prisoners based near the Thai-Myanmar border, said that Thet Win Aung was among more than 125 political prisoners who have died in Myanmar jails since 1988 because of abysmal conditions and brutal treatment.

The group said Thet Win Aung had been denied proper medical treatment, was also suffering from mental illness and was only allowed infrequent visits from family members. His elder brother, Htay Win Aung, a prominent pro-democracy activist, was arrested for the second time last month.

“The AAPP believes his death was not due to natural causes … he was systematically killed by the military regime,” it said.

The International Committee of the Red Cross has been denied access to Myanmar’s prisons since December last year. Earlier, it helped provide food and medicine to the 90 prisons and labor camps its representatives were allowed to visit.