Calcutta, Jan. 16: Blaze or blast, don’t expect any quick answers in the “do- it-now” capital called Calcutta.

A powerful explosion ripped through a pavement on Shakespeare Sarani this morning, injuring four pedestrians, tearing a tree, blasting a crater, twisting railings and spreading panic during office-goers’ peak hour.

However, as night fell several hours after the 9.45am blast, police had no clue what caused it in a city still licking the wounds of a fire that took over 100 hours to be brought fully under control.

The explosion took place in front of the Hindustan Unilever Building on 9 Shakespeare Sarani.

“The investigation is still on to ascertain the reason behind the explosion. We are looking at all possibilities,” said Jawed Shamim, deputy commissioner of police, detective department.

The epicentre was the base of a radhachura tree in front of the Hindustan Unilever office. Under the impact, a length of the bark was ripped off and a two-foot-deep hole was drilled at the foot of the tree.

“There was a deafening sound. The next moment, I saw the iron fence along the pavement lying in the middle of the busy road. The windowpanes of the nearby buildings cracked and some billboards in the area fell off,” said B.K. Singh, chief security officer, Nightingale Diagnostic and Medicare Centre.

It took him a while to realise what was happening. Singh rushed towards the tree — around 30 yards from the medical centre — and saw some people writhing in pain.

All the injured — Aditi Kumar Ghosh, 28, Mukul Saha, 49, Jaya Saha, 40, and Piyush Banerjee, 66 — were taken to Nightingale.

“All four were in a state of shock. Banerjee is in a critical condition with multiple injuries. We took out glass shards, stones and small metal pieces from their bodies,” a nursing home official said.

Not only were the windowpanes of the adjacent Nagaland House on 11 Shakespeare Sarani and the New BK Market on the opposite side of the road shattered, the explosion rocked the entire area.

“It was like a heavy object falling from a crane. The British Council windows rattled. We rushed out to see people running in fear,” said a BCL spokesperson, at the Camac Street office, a seven-minute walk from the explosion site.

The chaos that followed threw the rush-hour traffic on Shakespeare Sarani out of gear. “I was stuck on Theatre Road (the old name) for more than an hour. Someone told me that there was a bomb blast,” said businessman Sandeep Saha.

“Sitting in my Nagaland House room, I was reading a newspaper…. Suddenly, there was this explosion and it was so loud that my heart skipped a beat. I thought it must be some sort of a terrorist attack,” said Nohoshia, a health department official.

The “bomb” story spread like wildfire but the police ruled out the possibility.

“An explosion like this has not happened in the rec- ent past. We are seeking expert opinion,” said DCP Shamim.