New Delhi - Some of the hostages rescued from the weekend attack on an upscale restaurant in Bangladesh's capital were being questioned on Monday by investigators searching for clues about the possible masterminds behind the gruesome attack that left 28 dead, including many foreigners.Authorities were still holding five of the 13 hostages rescued when commandos stormed the restaurant in Dhaka's diplomatic zone on Saturday morning, killing six of the attackers and capturing one, according to officials.Bangladesh's police chief, AKM 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 authorised to speak to media about the ongoing investigation. The official said authorities were looking into the backgrounds of the five people and questioning their family and friends.It was not clear if all five were suspects, or if they were being held and questioned simply because authorities thought they might offer useful 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 also said the one of the attackers was a student in the same department at the university where the man teaches.The brutality of 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 the increasingly strident Islamist militants.Shift in tactics That the attackers targeted a popular restaurant in the heart of the diplomatic quarter of Bangladesh's capital signalled a shift in militant tactics. Previously, most attacks were carried out by gangs of young men wielding cleavers and machetes and hacking individual victims before fleeing.The English-language Daily Star newspaper on Monday said the bloody hostage crisis had left "the nation shattered and with a sense of extreme unease". The editorial also criticises authorities' consistent denial of the presence of any international terrorist groups, even as the Islamic State group claimed responsibility for the attack and released gruesome photographs that apparently depicted the torture of hostages."The methods employed and the savagery with which the killings were carried out are hallmarks of international terrorist outfits like ISIS and al-Qaeda. This is clear," the paper says in its editorial. "What is not clear is whether, after such overwhelming evidence of their presence, the official line will be one of denial?"Bangladesh police have said they are investigating whether the attackers had links with the extremist Sunni Muslim group based in Syria and Iraq. But the home minister insisted ISIS could not have guided the attack from abroad, as he said it has no presence in Bangladesh.Instead, Prime Minister Sheikh Hasina's government says the attacks are being carried out by domestic militants bent on imposing Islamic rule, and blames the political opposition of backing the attacks in order to create chaos in the country.Meanwhile, five Indian states that border Bangladesh were on high alert on Monday, with Indian police checking vehicles crossing the shared 4 000km border in case any militants were trying to flee, according to the head of India's paramilitary Border Security Force.On Monday, surrounded by tearful family members and a heavy security detail, Hasina and diplomats from Italy, Japan and other countries lay 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 US citizen, was also partially covered with a US flag.Another 17 hostages, nine Italians, seven Japanese and one Indian, were killed in the attack - many of them tortured with sharp instruments, according to police.Those bodies were to be flown back to their home countries on Monday. Family and friends of 19-year-old Tarishi Jain, meanwhile, gathered in the New Delhi suburb of Gurgaon as her body arrived for a ceremonial cremation.Two others who died were police officers who were killed on Friday night when police engaged the attackers in a gun battle at the start of the crisis.The stadium vigil was visited by hundreds of Dhaka residents, paying their respects to the victims.A Catholic Mass, Islamic prayer sessions and a candlelight vigil were held later on Monday.