History of the Anushilan Samiti
Around 1902, Aurobindo Ghosh arranged for Bannerjee to return to Calcutta to begin organisational work, where he began a gymnasium and a youth society. It was at this time that Bannerjee came in touch with Vibhuti Bhushan Bhattacharya, Sarala Devi Ghoshal and Pramathanath Mitra. In Calcutta, a few clubs and societies were still active at the time. Most notable was a gymnasium in Ballygunge Circular Road run by Sarala Devi herself, a second was an organisation named "Anushilan Samiti" founded by Satish Chandra Basu with the patronage of Pramathanath Mitra. A third one was a youth club called "Attōnnōti Samiti" (Self-improvement society) in central Calcutta.
History of the Anushilan Samiti
The two branches of "Anushilan" engaged at this time in a number of notable incidences of political assassinations and "dacoities" to obtain funds. In the meantime, in December 1907 the Bengal revolutionary cell derailed the train carrying the Bengal Lieutenant Governor Sir Andrew Fraser. In December 1907, Dhaka Anushilan Samiti assassinated D.C. Allen, a former district magistrate of Dhaka. In 1908, the activities of Anushilan saw eleven assassinations, seven attempted assassinations and bomb explosions, and eight dacoities in western Bengal alone. The targets of these "actions" included British police officials and civil servants, native Indian police officers, approvers and public prosecutors in cases of political crime, as well as the wealthy upperclass wealthy families. 
"Anushilan", notably from early on, established links with foreign movements and Indian nationalism abroad. In 1907, Barin Ghosh arranged to send to Paris one of his associates by the name of Hem Chandra Kanungo (Hem Chandra Das), he was to learn the art of bomb making from Nicholas Safranski, a Russian revolutionary in exile in the French Capital. Paris was also home at the time Madam Cama who was amongst the leading figures of the Paris Indian Society and the India House in London. The bomb manual later found its way through V.D. Savarkar to the press at India House for mass printing. This was, however, followed by a temporary stumble for "Anushilan". In 1908, two young recruits, Khudiram Bose and Prafulla Chaki were sent on a mission to Muzaffarpur to assassinate the Chief Presidency Magistrate D.H. Kingsford. The duo bombed a carriage they mistook as Kingsford's, killing two English women in it. In the aftermath of the murder, Khudiram Bose was arrested while attempting to flee, while Chaki took his own life. Police investigations into the murders revealed the organisations quarters in Manicktala suburb of Calcutta and led to a number of arrests, opening the famous Alipore Conspiracy trial. Some of its leadership were executed or incarcerated, while others went underground. Aurobindo Ghosh himself retired from active politics after being acquitted, his brother Barin was imprisoned for life. This was followed by the Dacca Conspiracy Case in 1909 which brought 44 members of the Dhaka Anushilan to trial. Anushilan, however, took its revenge. Nandalal Bannerjee, the officer who had arrested Kshudiram, was shot dead in 1908, followed by the assassinations of the prosecutor and the approver in the Alipore case in 1909.