Police in Beijing announced Tuesday they had arrested four people allegedly involved in the attack on Fang Zhouzi, a popular science writer known for his efforts in exposing academic fraud in China.

Two attackers disabled Fang with pepper spray, then hit him with a hammer, on a street near his home after a TV interview in Shijingshan District on Aug. 29. Fang suffered slight injuries in the attack.

With the help of two witnesses and surveillance video, police arrested three men in early September following another attack on Fang Xuanchang, an editor of the financial journal Caijing, on June 24.

The three confessed that Xiao Chuanguo, head of the Urology Department of Wuhan Union Hospital, hired them for the attack.

Xiao was arrested at the Shanghai Pudong airport on Tuesday.

Xiao confessed to the police that the attacks were revenge, as he believed the exposure of his academic fraud by Fang Zhouzi and Fang Xuanchang, another anti-fraud activist, made him fail in his bid to become a member of the Chinese Academy of Sciences.

Xiao said he hired Dai Jianxiang, 45, and Dai arranged for two younger men, Long Guangxing, 31, and Xu Lichun, 32, to commit the attacks.

Fang Zhouzi was born in Fujian Province in 1967. He is well known for discovering bogus research and academic fraud in China, including the latest cases of Li Yi, a Taoist priest in southwest China' s Chongqing Municipality, and Jun Tang, former CEO of Microsoft Greater China Region.