Student leaders were put under close surveillance by the authorities, traffic cameras were used to perform surveillance on the square and the restaurants in the nearby area and where students gathered were wiretapped.[108] This surveillance led to the identification, capture and punishment of participants of the protest.[109] After the massacre, the government did thorough interrogations at work units, institutions and schools to identify who had been at the protest.[110]
https://en.wikipedia.org/wiki/1989_Tiananmen_Square_protests...