Another constable was injured in the pre-dawn incident and policeman J B Subba, who allegedly opened fire with his service rifle after attacking his colleagues with an axe, was detained for questioning by Delhi Police.

Initial investigations revealed that the incident could have been the result of a brawl between the personnel from an India Reserved Battalion. The Dena Bank they were guarding is located near Golcha cinema hall and opposite the office of the deputy commissioner of police at Daryaganj in central Delhi.

"We got a call about the firing in the bank at around 4.52 am. When police teams reached the spot, five personnel of Sikkim Police were found dead. Investigations are at a preliminary stage," DCP (Central) Alok Kumar said.

Delhi Police questioned the injured constable, identified as Nari Lepcha, and several others, including newspaper vendors and the guard of a nearby ATM, who heard the gunshots.

Lepcha suffered gunshot injuries to his leg and shoulder, and was admitted to LNJP Hospital. The dead policemen were identified as Shantiveer, Vishal Tiwari, Karma Bhutia, Lakshman Subba and Kumar Gislet.

Delhi Police refused to confirm whether J B Subba had been arrested, though Sikkim Police officials said he had been taken away for questioning. It is also not immediately clear why Subba had attacked his colleagues but police officials ruled out robbery as a possible motive for the incident. The Sikkim Police officials said they were assisting the city police in their probe.