Investigators hope the escaped extremist and another man who was arrested during the major gunfight in Dhaka yesterday will shed light on the group's proclaimed ties with the IS.
"We've alerted all checkpoints in the capital in a bid to arrest him (the escapee)," deputy commissioner of Dhaka Metropolitan Police, Masud Ahmed, told AFP.
Officers believe the nine slain extremists were part of the same group that killed 22 people during an attack on an upscale Dhaka cafe on July 1 -- an attack claimed by IS.
"We're conducting an investigation. Hasan has claimed that they were IS members," a senior security official told AFP, referring to a 25-year-old arrested in the raid who is being treated in hospital.
"We suspect he is one of the leading members of the group."
Investigators are also trying to retrieve information from a laptop and several mobile phones recovered in the raid.
IS has claimed responsibility for dozens of murders of religious minority members and foreigners in Bangladesh in recent months.
Following the Dhaka cafe attack, IS released photos of five gunmen posing with the group's flag. The group also published gruesome images of the carnage before commandos ended the siege.
Meanwhile, the British Council temporarily closed its offices in Bangladesh Wednesday over safety fears, as a rumour swirled that Islamist extremists would soon target a major market, school or foreign organisation.
"The safety and security of our staff and customers is always our top priority," Barbara Wickham, director of the British Council in Bangladesh, said in a message on its website.
Many schools and businesses run by the nation's small Christian community closed their doors for days last week fearing an attack from extremists, while several parents have kept their children home from school.