Bangladeshi police hunted on Tuesday for six members of a domestic Islamist group they suspect helped gunmen attack a Dhaka cafe.
Officials have also begun questioning families of the militants for clues as to what turned them into killers.
Gunmen stormed the restaurant in Dhaka's diplomatic zone late on Friday and killed 20 people, most of them foreigners from Italy, Japan, India and the U.S., in an assault claimed by Islamic State.
It was one of the deadliest militant attacks in Bangladesh, where Islamic State and al Qaeda have claimed a series of killings of liberals and members of religious minorities in the past year.
The government has dismissed those claims, as it did the Islamic State claim of responsibility for Friday's attack.
Police believe that Jama'atul Mujahideen Bangladesh (JMB), an outlawed domestic group that has pledged allegiance to Islamic State, played a significant role in organising the band of privileged, educated young men who carried out the attack.
"Six members of JMB have been shown as accused in the case. We are trying to arrest them because they could be the mastermind," Saiful Islam, a senior police officer involved in the investigation, told Reuters.
The JMB has been accused of involvement in many of the killings over the past year and Islam said police were interrogating more than 130 of its members already in custody in the hope of gleaning clues.
"We don't know who is the mastermind behind the attack. We just know that these boys were guided to launch an attack on the restaurant," he said.