The British released a lot of details on how the attack was carried out, including the use of promises (quickly broken) to allow demonstrators (including survivors of the first round of killing) to leave. The troops apparently had orders to kill all civilians in the square and destroy the bodies where they fell. This included crushing the dead using armored vehicles, burning those remains and flushing those remains down storm drains. The area was sealed off for over a month so the cleanup could be thorough.
After the June 4 massacre the Chinese government enacted a growing number of measures to erase the June 1989 demonstrations (there were over 400 different demonstrations across the country with millions of Chinese participating) from popular memory. Martial law was imposed on the capital until 1990 to make it easier to hunt down survivors and terrorize anyone inclined to discuss the massacre.