Why was Ai Weiwei assaulted by police in Chengdu in 2009?

In 2009, the Chinese artist and activist Ai Weiwei was assaulted by police in Chengdu while he was in the city to support a fellow activist who was on trial.1

At the time, Ai had been conducting a "citizen investigation" into the 2008 Sichuan earthquake, specifically seeking to hold officials accountable for the deaths of thousands of children in school buildings that collapsed due to suspected shoddy construction. His efforts to document these fatalities and his public criticism of the government frequently led to state-sponsored repression, including surveillance and physical confrontation.12