NEW DELHI » The bodies of the seven Japanese killed in a militant attack in Bangladesh returned to home Tuesday as investigators in Dhaka searched for clues about the possible masterminds behind the gruesome attack that left 28 dead.

A Japanese government plane took the bodies back to Tokyo’s Haneda Airport, where the bodies, in boxes covered with white cloth, were lowered slowly in pairs from the high cargo bay of the 747 and lined up on the tarmac.

Japanese Foreign Minister Fumio Kishida, Bangladesh Ambassador Rabab Fatima and other officials laid bouquets of flowers in between the boxes.

In Dhaka, authorities were still holding five of the 13 hostages rescued when commandos stormed the restaurant in Dhaka’s diplomatic zone Saturday morning, killing six of the attackers and capturing one.

Bangladesh police chief A.K.M. Shahidul Haque said authorities would be interrogating two men, including a suspected militant, who were detained during Saturday’s operation. He would not say whether either had been among those counted as hostages, only that they were being treated in a hospital for unspecified injuries.

A second official said the five former hostages still being held included a Canadian citizen of Bangladeshi origin and a Bangladesh-born British citizen. The official spoke on condition of anonymity, as he was not authorized to speak to the media about the ongoing investigation. The official said authorities were looking into the backgrounds of the five people and questioning their families and friends.

Global Affairs Canada, the government department which manages Canada’s diplomatic and consular relations, and consular officials at the High Commission of Canada in Dhaka said they had not received any reports of a Canadian citizen being detained.

It was not clear if all five were suspects, or if they were being held and questioned simply because authorities thought they might offer information in tracing the origins of the attack.

The official confirmed investigators were also speaking with a third man described by local media as a Bangladeshi who was trapped inside the restaurant along with his wife and two children. The man, a teacher at a private university in Dhaka, had returned to Bangladesh recently after living nearly 20 years in Britain.

Some photographs and several crude videos taken from an apartment near the Holey Artisan Bakery show the man talking to someone while attackers allowed him to leave before paramilitary forces launched the rescue operation on Saturday. The man’s friends and police said that one of the attackers was a student in the same department at the university where the man teaches.

The attack – the worst convulsion of violence yet in the recent series of deadly attacks to hit Bangladesh – has stunned the traditionally moderate Muslim nation and raised global concerns about whether it can cope with increasingly strident Islamist militants.

That the attackers targeted a popular restaurant in the heart of the diplomatic quarter of Bangladesh’s capital signaled a shift in militant tactics. Previous attacks were carried out by gangs of young men wielding cleavers and machetes and hacking individual victims before fleeing.

Bangladesh police have said they are investigating whether the attackers had links to the Islamic State group, though the home minister insisted IS had no presence in Bangladesh and could not have guided the attack. The government has blamed the restaurant attack and other recent killings on domestic militants bent on imposing Islamic rule. IS claimed responsibility for the attack.

The Italian Foreign Ministry, after nine Italians were killed in the restaurant, posted travel advisories saying it could not exclude the possibility of further attacks in Bangladesh. The advisory urged people to exercise the “utmost prudence,” particularly in places frequented by foreigners, and to limit their activities to only what was necessary.

On Monday, surrounded by tearful family members and a heavy security detail, Prime Minister Sheikh Hasina and diplomats from Italy, Japan and other countries laid flower wreaths beside the coffins holding the three Bangladeshi victims.

The coffins were draped in the Bangladeshi flag – a red disc on a green background. That of Emory University student Abinta Kabir, a Miami resident whose family confirmed she was a U.S. citizen, was also partially covered with a U.S. flag.

Two police officers and 17 other hostages – nine Italians, seven Japanese and one Indian – were killed. ontributed to this report.