Bengal CID Friday took over the case of killing of four people in firing by the CISF during polling in Cooch Behar district on April 10, a senior government official said.
A Special Investigation Team (SIT) of the Criminal Investigation Department (CID) has been constituted to conduct the probe in the incident which happened during the fourth phase of polling at Sitalkuchi.
The incident had set off an acrimonious war of words between the ruling TMC and challenger BJP, and triggered public outrage.
Investigators will visit booth number 126/5 in Jorpatki in Sitalkuchi assembly constituency where CISF personnel had opened fire after they allegedly came under attack from a group of villagers.
The TMC has claimed those killed were its supporters who were gunned down when they were standing in queue to cast their votes. Chief Minister Mamata Banerjee, who is also the TMC chief, had called the incident a "genocide".
Besides talking to witnesses and recording their statements, the SIT will also go through video footage of the incident. A video clip has emerged on social media, the officer said, adding a forensic test will likely be conducted to establish its authenticity.
The SIT will also talk to officers and personnel of the local police station who rushed to the scene soon after the incident.
The Election Commission had cancelled the voting at the polling station after the incident and imposed a ban on entry of politicians into Cooch Behar for 72 hours apprehending trouble.
Mamata had visited Sitalkuchi on Wednesday and met the families of the victims.