He said he was going to pray at a mosque and left behind his wife and child at their Baridhara home in Dhaka on Mar 4, 2015, said his sister Zannatul Mawa.
Raja, described as quiet and good-natured by his family, completed his studies in Australia.
He is seen speaking in a propaganda video from inside an armoured vehicle before taking part in a suicide bombing in Iraq’s Tikrit.
His family, from Chittagong’s Hathazari, lived at Baridhara DOHS.
Raja once lived in Zambia where his father, AKM Kamal Uddin, worked in the Gambia Union Bank. He sat for this O-levels examination from the West African nation.
His father then returned home to join a top job at a private bank.
The youngest among three siblings, he sat for his A-levels after studying in Chittagong’s Sunshine Grammar School and College.
“He returned home in 2010 and got married in 2011. Our father fell ill and died that year.”
He seemed more quiet upon his return from Australia, she said. “He prayed regularly. We didn’t see anything wrong with that then.”
She said a police complaint was filed after Raja 'went missing' in 2015.
"We later heard about him joining a militant group," she said and added that she spoke to him over phone twice between April and October in 2015.
"He only said that he was fine and asked us not to worry about him. We said those to the law-enforcing agencies. We couldn't imagine that such a quiet boy would become an extremist," Zannatul said.
The family identified him when terrorism monitoring group SITE Intelligence said IS' affiliate Furat Media released a 'posthumous video of the Bangladeshi 'suicide bomber'.
It did not say anything about the time and place of Raja's 'death'.
Australia's newspaper The Australian said Raja was identified as “Abu Maryam al-Bengali” in the video. He speaks in English to call on others to take up jihad in the video.
RAB-7 commander Lt Col Miftah Uddin Ahmed said, "He went missing from Dhaka after completing A-level here in Chittagong. We've learnt that only one of his sisters live here."