The Calcutta Police has registered an unnatural death case over the death of singer KK, agencies and multiple news portals have reported.

The case was registered at the New Market police station. A senior officer of the Calcutta Police told PTI that a post-mortem would be conducted on Wednesday to ascertain the exact reason for the death.

"We have started an investigation into singer KK's death and a case of unnatural death has been registered with the New Market Police Station. We are talking to the hotel authorities and scrutinising CCTV footage to understand what had happened before he was taken to the hospital," the police officer added.

According to police sources, his face and head bore injuries. The autopsy would be conducted at SSKM Hospital. The police will check the CCTV footage at Grand Hotel and talk to the hotel staff and the event organisers, reports IndiaToday.

The 53-year-old singer was rushed to CMRI hospital after he fell ill while performing on Tuesday ngiht. Doctors there declared him "brought dead".

His wife and children will reach Calcutta on Wednesday morning.

Krishnakumar Kunnath, popularly known as KK, is survived by his wife and two children. He performed at a concert organised by Gurudas College at Nazrul Mancha in south Calcutta for almost an hour in the evening.

He was "feeling heavy" after reaching his hotel, and soon collapsed, officials said.

He was taken to a nearby private hospital where doctors declared him brought dead, they said."KK was brought to the hospital around 10 pm. It's unfortunate that we could not treat him," a senior official of the hospital said.

Doctors at the hospital said they suspected "cardiac arrest" to be the cause of the death.

Social media users have said that the South Kolkata venue, Nazrul Manch, is a closed auditorium and was overcrowded for the concert, reports NDTV.

Visuals show the singer sweating profusely as he takes a break to wipe his face. Other voices can be heard saying, "Bohit zyada garam hai".

At one point, KK is seen gesturing to a man on stage and it appears that he is talking about the air-conditioning.

While the Nazrul Mancha has a capacity of about 2,400, social media users claimed that about 7,000 people had entered the venue where KK was performing for a college fest.

A video from the venue shows a staircase in which a group of students appear to be playing with a fire extinguisher. Many have shared this video, questioning the role of organisers.

KK, who was on a two-day tour to Kolkata, also performed at Nazrul Manch on Monday evening in a concert organised by Vivekananda College. He was supposed to return to New Delhi on Wednesday.