Dhaka: Bangladeshi police said on Tuesday one of the men they shot dead during the siege of a Dhaka cafe on the weekend may have been a hostage killed by mistake, while the hunt for accomplices of the gunmen who killed 20 people focused on six suspects.

Police on Tuesday named five Bangladeshi gunmen who stormed the restaurant in Dhaka's diplomatic zone late on Friday. Most of the victims in the violence claimed by Islamic State were foreigners, from Italy, Japan, India and the United States.

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.

Pictures of five young men clutching guns and grinning in front of a black flag were posted on an Islamic State website hours after the attack, along with the claim of responsibility, but despite that, authorities have ruled out a foreign link.

Police believe that Jamaat-ul-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 attackers.

Confusion over exactly how many gunmen were involved was at least partly cleared up on Tuesday when police said among the six people security forces killed when they stormed the building to end a 12-hour stand-off was Saiful Islam Chowkidar, a pizza maker at the Holey Artisan restaurant.

"We killed six people in the restaurant. A case has been registered against five. The sixth man was a restaurant employee," Saiful Islam, a top police official investigating the attack, told Reuters.

"He may not be involved," he said, adding that the investigation was going on.

An employee of the cafe, shown a photo of a man killed at the eatery and wearing a chef's outfit, identified him as Chowkidar, and said he had worked there for 18 months.

Police named five men as attackers in a case filed on Tuesday to allow them to launch official investigations, including questioning families of the militants for clues as to what turned them into killers.

Two other suspects are in hospital.

'GUIDED'

Police said they were hunting for six members of the JMB who were suspected of organising 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," Islam said.

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.

The five named in the case filing were Nibras Islam, Rohan Imtiaz, Meer Saameh Mubasheer, Khairul Islam and Shafiqul Islam.

The attack marked a major escalation in the scale and brutality of violence aimed at forcing strict Islamic rule in Bangladesh, whose 160 million people are mostly Muslim.

It has shocked the country, as have details emerging about the well-to-do lives of some of the gunmen.

At least three of the gunmen were from wealthy, liberal families who had attended elite Dhaka schools, in contrast to the traditional Bangladeshi militant's path from poverty and a madrassa education to violence.

Three of the attackers had been missing since the beginning of the year, police have said.

Two had attended a private university in Malaysia, one of whom, Nibras Islam, was not particularly religious, according to a student who played football with him at a private college in Dhaka between 2009 and 2011.

"We are in touch with investigators in Malaysia and they are sharing all the information but as of now we have not found any links with international militant groups," Islam said.

One of the dead gunmen was from a poor family and had studied at a madrassa and another hailed from a lower-middle class background, said another senior police official who declined to be identified.