BEIJING (AP) — Bao Tong, a leading voice for political reform in the Chinese Communist Party who was purged after the bloody 1989 Tiananmen Square crackdown, has died at age 90.

Son Bao Pu said in a tweet that his father passed away peacefully Wednesday morning. Bao had been living in a Beijing suburb under tight police supervision.

Bao Tong joined the Communist Party as an underground member prior to its 1949 seizure of power under Mao Zedong, and rose to be a top aide to former Communist Party general secretary Zhao Ziyang, who was deposed after expressing support for the student-led protests that called for more personal rights and an end to corruption and dictatorship.

After the June 3-4, 1989, army assault on the protesters, in which hundreds or possibly thousands were killed, Zhao was placed under house arrest, while Bao spent most of the rest of his life in prison or under some form of detention. Bao Pu moved to Hong Kong where he published works on the inner workings of the Communist Party, prior to a sweeping clampdown on free speech in the semi-autonomous Chinese city.

Bao Pu’s Hong Kong-based company is best known for publishing a posthumous series of interviews with Zhao.

Along with Bao Pu, now a U.S. citizen, Bao Tong leaves a daughter, Bao Jian.