A sum of Rs 4 lakh, kept aside for distribution among pensioners, went missing at the Baranagar sub-post office on Monday morning.

Pensioners queueing up in front of the post office saw a closed door at 10am and found the postmaster inside, gagged and tied to his chair.

“Someone pushed open the door, which was never closed during office hours, after hearing a muffled sound,” said Ashish Gupta, a pensioner in the queue.

“The postmaster, Bimal Ghosh, was tied to the chair and the iron chest in which he kept the money was empty,” said Gupta.

Two criminals had walked into the post office at 9.30am and overpowered him, the postmaster told his rescuers.

The goons tied him to the chair, and decamped with the money in the space of a few minutes, said Ghosh.

Police, however, are sceptical of Ghosh’s account. “A sum of Rs 4 lakh doesn’t often make its way to a sub-post office. The heist was planned by people who knew the money had arrived,” said Praveen Kumar, superintendent of police (North 24-Parganas). “There are insiders involved.”

“We have detained the postmaster for interrogation and are checking why there was no security arrangement in the post office,” said Kumar.

The police have summoned senior officers of the postal department to find out the exact amount of money that was robbed from the chest. “Investigations are on, and we hope to solve the case soon,” said the police superintendent.