A group of mothers whose children were killed in the 1989 protests in Tiananmen Square have renewed calls to China's leaders for a full investigation as the 20th anniversary approaches.

The Tiananmen Mothers — a group of mothers and relatives who lost family in the June 4 pro-democracy crackdown — called for an official investigation, compensation to the victims' families and punishment for those responsible.

In the open letter made public Friday, Chinese leaders were also urged to "break the taboo" of the demonstrations and seek the truth behind the military crackdown against student-led protesters.

China's leaders sent tanks and troops to crush the peaceful protests on June 3-4, which were deemed a threat to Communist Party rule. Hundreds — possibly thousands — of people were believed to have been killed, but the government has never allowed for a full accounting of the events.

"This will require each deputy to demonstrate extraordinary courage and resourcefulness, political courage and wisdom, to break the taboo and face head-on the unspeakable tragedy that took place 20 years ago and resolve "June Fourth" with the truth," said the letter, referring to delegates to the annual legislature.

The activist group has released a version of the letter every year for two decades ahead of the annual session of the National People's Congress, which starts March 5.