Three security men of the India Reserve Battalion (IRB) were killed in Jharkhand's Maoist-hit Gumla District on Thursday.
They were killed by a colleague, who opened fire on them after getting into an argument.
The incident took place during poll duty hours.
IRB Commandant Surya Pratap Mandal said Constable Bikas Tiwari started firing frantically on his colleagues following an argument. Mandal said Tiwari first killed one person, and when he saw others moving towards him, he killed two more.
Tiwari has since surrendered before the police, which is investigating the cause behind the incident.