Nankana Sahib is the birth place of Guru Nanak Dev Ji. At the time of this incident, this place was being managed by Mahant Narayan Dass.
A Jatha of 150 GurSikh reformers, led by Bhai Lahshman Singh, were visiting Nankana Sahib to seek Gurdwara reforms and it’s liberation, through non-violent and peaceful means.
However, the managing Mahant had hired 28 Pathans and goondas from Majha, collected arms and ammunition besides other weapons, which caused a scare in the surrounding areas. He attacked the unscheduled Jatha, fired upon them without any warning and hounded them from room to room.
At least one of them was tied to a tree and burnt alive. When a massacre inside the shrine was on, a group of Sikh devotees arrived outside.
Mahant Narain Das on horseback ordered the killing of each and every long haired Sikh, and his men pursued some of them in the fields up to the railway station, killing and burning most of them. He also tried to burn down the dead bodies in a group inside the shrine.
Those struck outside were thrown into kilns or burnt alive. The Guru Granth Sahib was riddled with bullets. This incident sent a wave of indignation among the Sikhs and other people throughout the world. The holy shrine of Nankana Sahib was eventually liberated. Additionally, this single incident triggered off a Gurdwara liberation movement throughout Punjab.
The main objective of this movement was to consolidate the management of all historical Sikh Shrines under a single elected body which came to be known as the Sharomani Gurdwara Prabandhak Committee or SGPC for short.
Sessions Court Judgement on Oct. 12, 1921, sentenced the Mahant and seven others to death, 8 to transportation for life, 16 Pathans to 17 years rigorous imprisonment, and acquited the remaining sixteen.
However, the High Court, on March 1922, reduced the sentence for the Mahant to transportation for life, confirmed death sentence only for three, transportation for life against two and acquited all others including the Pathans. Source Mahan Kosh