Zhou Yongkang, a top security official during former President Hu Jintao's regime, was being probed along with his relatives for corruption.
Zhou was a member of the 17th Politburo Standing Committee, China's highest decision-making body.
Zhou was arrested and expelled from the ruling Communist Party last month, becoming the most senior communist official to face prosecution since the 1980s.
More than 53,000 officials from various departments, including former vice chairman of the Military Commission Xu Caihou were also being probed.
The SSP, however, did not elaborate how many of those being probed were prosecuted.
Meanwhile, Liao Shaohua, a former Communist Party chief of Zunyi City in Guizhou province, today confessed in the Xian Intermediate People's Court that he accepted bribes totalling more than 13 million yuan (USD 2.1 million).
Liao admitted taking the money to help various companies between 2004 and 2012. "I turned out this way because I couldn't exercise self-restraint."
The SPP has said it would strengthen investigating officials involved in areas such as the major construction projects, land transfers and mineral resources exploitation, state-run Xinhua news agency reported.
The SPP also announced it would establish a new "general administration for the graft fight" in 2015, upgrading its anti-corruption bureau set up in 1995.
