BEIJING (AP) — China’s Communist Party says it has launched an investigation into a former domestic security chief who was once among the country’s most feared leaders.

The official Xinhua News Agency cited the party as saying Tuesday it is investigating Zhou for serious violations of party discipline, but did not provide details.

Until his retirement in 2012, Zhou was one of nine leaders in the party’s ruling inner circle, whose members had until now been considered off-limits for prosecution in an unwritten rule aimed at preserving party unity.