On the evening of Saturday, two bombs exploded at one end of the street near the building in Shibbarhi on the outskirts of the city.
The blasts killed six, including two police officers, and injured at least 40 people, including law enforcers. The RAB intelligence chief has been also badly injured.
A case over the incident was filed early on Monday with Moglabazar Police Station of the Sylhet metro police.
OC Khairul Fazal told bdnews24.com that the case accuses unidentified persons. "The case details do not state any number of suspects. We have launched investigations from Sunday."
Army commandos took charge of the operation from the police on Saturday and began the assault on the complex housing a five-storey and a four-storey building.
At a media briefing in the evening, the army said 78 people trapped inside the building complex were evacuated.
Soon after the army's media briefing, two blasts at one end of the street to the building left six persons, including two police officers dead.
At least 40 others, including the intelligence wing chief of RAB, were injured in the blast.
Police are yet to make an official statement over the blasts, but Sylhet metro police's Additional Commissioner Md Rukon Uddin said they suspect militants behind it.