BEIJING--Three top military officers who are members of the powerful Central Military Commission are being questioned over unspecified improprieties, according to Communist Party sources.

One of them is Gen. Fang Fenghui, 66, who previously served as chief of the Joint Staff Department of the People's Liberation Army. One source said Fang is being held in isolation at a Beijing military facility.

The possibility of Fang and the two others being stripped of their positions as a result of the investigation suggests that Chinese President Xi Jinping is moving to further tighten his grip over the military ahead of the Chinese Communist Party's National Congress scheduled to convene in October.

If the three are removed, it would mark the first purge of Central Military Commission members under Xi's rule.

The sources said it was highly unusual for a trio of top officers to simultaneously face questioning.

According to sources, the two other officers apparently being questioned are Gen. Zhang Yang, director of the General Political Department, and Adm. Wu Shengli, the former commander of the Chinese Navy.

Fang, Zhang and Wu are among the 11 members of the Central Military Commission, which is headed by Xi.

Because the three officers all are at the top of the military pyramid in their respective fields, something highly unusual would seem to be going on, especially if disciplinary action is taken against them.

Xi has long focused on rooting out corruption, and in 2015 the top military officer at that time, Guo Boxiong, was stripped of his rank after being accused of accepting bribes.

However, the wholesale questioning of three top officers could trigger resistance among the military.

Fang served as commander of the former Beijing military region before he was promoted to the Central Military Commission in 2012.

He accompanied Xi when he visited the United States in April for his first summit meeting with U.S. President Donald Trump. Fang represented the Chinese military during a June meeting intended to encourage dialogue between those in charge of foreign affairs and national security in the United States and China.

Fang was considered an expert in cyber-security and military strategic matters. Some pegged him as a future vice chairman of the Central Military Commission.

However, rumors first emerged Aug. 26 that Fang might be in trouble when he did not attend a meeting of top military officers of China and Pakistan.

Gen. Li Zuocheng represented China as the chief of the Joint Staff Department instead.