The explosion took place at a construction material dump in the Pan Bazar area when the four passersby, including a woman, were injured around 11.45 am, the sources said.
"We don't suspect it to be a bomb explosion as there were no splinters found and the injuries were due to stones hitting them," said DCP Ranjan Bhuyan who is at the site.
"We don't suspect it to be a sabotage or terrorist activity from the nature of the blast. It happened in a dump of sand kept for roadside drain construction. It may have had an unexploded shell which went off today," the DCP said.
An expert bomb squad would be arriving soon, he added.
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