Empress Zhangsun
Later in 643, Li Chengqian the Crown Prince (the oldest son of Emperor Taizong and Empress Zhangsun, who had died in 636), locked into an intense rivalry with his younger brother Li Tai the Prince of Wei, who was also born of Empress Zhangsun and was Emperor Taizong's favorite son, was discovered to have plotted to overthrow Emperor Taizong, along with Hou Junji and Emperor Taizong's sons-in-law Zhao Jie (趙節) and Du He (杜荷, Du Ruhui's son). Emperor Taizong put the investigation in the hands of Zhangsun, Fang Xuanling, Xiao Yu, Li Shiji, and the officials in charge of the supreme court and the legislative and examination bureaus of government, and they determined that, indeed, Li Chengqian had plotted to overthrow Emperor Taizong. Li Chengqian was deposed, while his coconspirators were executed.