SMP Additional Commissioner SM Rukon Uddin replied to reporters' queries after a media briefing by the army on Monday.
"A bag was left on the road with the bombs in it. No one hurled the bombs," he said.
Asked how the blasts occurred amid heightened security in the area during the raid, Rukon said, "The area was very dark due to a power outage, and a huge number of people gathered here to know about the raid. The attackers' took the chance."
Middle East-based radical group Islamic State reportedly claimed credit for the attack.
Rukon said police traced owners of four of the five motorcycles lying at the blast site. Police asked BRTA for information on the other one, he added.
The army said in the briefing that four militant suspects were found dead in the raided building. The operation was not called off yet because the militants planted explosives all over the building, the army said.
In reply to a query how the militants stored so much explosives at the den, he said, "This city is not secured. The alleys are not under CCTV surveillance. Moreover, the people have constructed buildings without following the code."
"And they (militants) did not brought those (explosives) at a time. Those could be brought at different times," he added.
The additional commissioner of SMP also advised the residents of the city not to panic.
"Such incidents took place in Chittagong, Dhaka's Holey Artisan, Dinajpur. The attack (in Sylhet) was carried out in continuance of those incidents.
"But police are active. The militants will be traced through investigation so that they cannot spread," he said.
Asked when the situation will normalise in the area, he said, "As the operation is at the last phase, I hope we will be able to give the good news tomorrow (Tuesday)."