Bangladesh police sought more information on Monday from friends and family of the men suspected of carrying out a deadly attack on a restaurant in the capital, and some are believed to have attended top schools and colleges at home and abroad. The 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 United States, in an assault claimed by Islamic State. It was one of the deadliest militant attacks to date in Bangladesh, where Islamic State and al Qaeda have claimed a series of killings of liberals and religious minorities in the last year while the government says they were carried out by local groups. Whoever was responsible, Friday's attack marked a major escalation in the scale and brutality of militant violence aimed at forcing strict Islamic rule in Bangladesh, whose 160 million people are mostly Muslim. Islamic State posted pictures of five fighters it said were involved in Friday's atrocity to avenge attacks on Muslims across the world. "Let the people of the crusader countries know that there is no safety for them as long as their aircraft are killing Muslims," it said in a statement. Posts on Facebook identified the men, pictured on an Islamic State website grinning in front of a black flag, as Nibras Islam, Rohan Imtiaz, Meer Saameh Mubasheer, Andaleeb Ahmed and Raiyan Minhaj. Most went to prestigious schools or universities in Dhaka and Malaysia, officials said. "A majority of the boys who attacked the restaurant came from very good educational institutions. Some went to sophisticated schools. Their families are relatively well-to-do people," Bangladeshi Information Minister Hasanul Haq Inu told India's NDTV. TRACING ROOTS Several posts on social media said the man identified by police as Nibras Islam attended Monash University in Malaysia. A friend who knew him while he studied at Dhaka's North South University told Reuters that Islam later went to Monash. Two others went to an elite public school in Dhaka called Scholastica. Masudur Rahman, deputy commissioner of Dhaka police, said officers were investigating those links. Initial evidence points to the fact that Nibras Islam, Meer Saameh Mubasheer and Rohan Imtiaz "may be involved," in the attack, Rahman said. "There may be a link to international terrorist groups, including IS; we are looking into that angle," he said, adding that the attackers were educated, well off and brainwashed. Saifaul Islam, another investigator, said police were holding two people suspected of involvement in the assault, including one detained soon after the attack. "We have two persons with us, but we don't know if they are victims or suspects. They are currently undergoing treatment and we'd get to know about their role in the incident only after they recover." Nobody had yet come forward to claim the bodies of the six dead men, he said. "We are taking DNA samples of them and will see if it matches with the families. We have some suspicions, we know some boys had gone missing over the last two-three months." Just days after the attack claimed by its rival jihadi movement Islamic State, a regional branch of al Qaeda urged Muslims in India to revolt and carry out lone wolf attacks. The call by al Qaeda in the Indian Subcontinent (AQIS) follows warnings by security officials and experts that the two groups are trying to outdo each other in the region and claim the mantle of global jihad. Bangladesh's $26 billion garment industry is braced for the fallout from the killings, fearing major retailers from Marks and Spencer to Gap Inc could rethink their investment. Japan's Fast Retailing Co, owner of the Uniqlo casual-wear brand, will suspend all but critical travel to Bangladesh and has told staff there to stay home. U.S. Secretary of State John Kerry has offered Bangladeshi Prime Minister Sheikh Hasina help in the investigation. "The Secretary (Kerry) encouraged the government of Bangladesh to conduct its investigation in accordance with the highest international standards and offered immediate assistance from U.S. law enforcement, including the FBI," U.S. State Department spokesman John Kirby said.