The copper factory, in Wellampitiya, a north-eastern suburb of Colombo, belonged to 'calm and devout' Inshaf Ahamad, who is understood to have blown himself up at a hotel frequented by foreigners in the atrocity.
Inshaf's brother-in-law said the businessman drove a brand new white Land Cruiser and came from a middle class background.
'We had no idea what they were planning. If we had, we would have immediately told the police.'
Inshaf is said to have attacked the Cinnamon Grand hotel where retired fireman Bill Harrop and his wife Sally were killed, while his brother Ilham targeted the Shangri-La, massacring British lawyer Ben Nicholson’s wife and two children.
Their father, a successful spice trader, was arrested at the family home after bombs went off at the family home, killing three police officers.
Ashkhan went on: 'We feel no emotion apart from anger. Pure anger. They were successful men with everything to live for.
He said his sister, Ishfan's wife, was 'living in a dream world' following the massacre.
'She can't believe it really happened. She is in great psychological pain.'
After obtaining the material for the bomb, Inshaf, a father-of-four in his mid-thirties, told his wife that he was going to Zambia on a business trip, Mr Alamdeen said.
She dropped him off at the airport on Friday in time for an 6:50pm flight. They had been married for 10 years.
'It was not a normal goodbye,' Mr Alamdeen recalled. 'My sister said he seemed shifty. He was tapping his neck and acting strangely. Then out of the blue he told her to be strong.'
The terrorist, who had eight siblings, phoned his wife one last time at 7:30am on Sunday, an hour before the attack, to say his final goodbyes.
'He told her that he was in Zambia and everything was fine,' Mr Alamdeen said. 'Then an hour later he was dead.'
Inshaf's copper factory was raided last night by police, who took nine suspects into custody, including the manager, supervisor and technician.
The investigation is ongoing, but detectives believe the location was used to build the suicide vest using the explosive substance triacetone triperoxide, dubbed 'Mother of Satan' by Al Qaeda for its destructive power.
The substance, a calling card of Islamist terrorists, played a role in both the 2017 Manchester bombing and 2015 Paris Bataclan attacks.
When MailOnline visited the factory, about 11 Indian and Bangladeshi immigrants were there, saying they felt 'very nervous'.
'The police have come here many times and looked at everything,' said Mohammad Sarowan, 25, from Bangladesh. 'We have done nothing wrong. We are innocent. They are interested mainly in the Sri Lankans.
'This factory is our life. We live here and work here and we are paid $150 a month. Now the boss has gone and police are here every day.'
Inshaf was 'calm and devout' but never wore traditional Muslim dress, Mr Sorowan added.
Tall and lightly bearded, he would come to the factory daily for about 20 minutes and speak only to the manager.
'He never allowed anybody to take his photograph and he said it was because of his religious beliefs,' the Bangladeshi migrant said.
A senior police source closely involved with the investigation told MailOnline that Inshaf's financial background 'doesn't appear to add up'.
'Where did he get so much money from? That is the question we are asking,' the source said. 'The network is so widespread that we are just starting to piece it together.
'It's like a cancer that has already spread through the body that we are trying to cure.'
Investigators believe that Inshaf was acting with foreign terrorist help, the police source added.
Inshaf's brother-in-law said that he last saw the bomber three days before the attack when he came over for a dinner of coconut roti.
'We had a conversation about going camping. There was nothing strange about him at all,' he said.
'He was no more religious than anyone else and was just a perfect guy with everything to live for. His kids are aged eight, six, four and two.
'Even now, I don't want to believe it. Who would expect something like this in their family?'
Born in the city of Kandy in central Sri Lanka, Inshaf attended the respected D. S. Senanayake College in Colombo. He lived with his family in an apartment in the Dematagoda district of Colombo owned by his father.
It was this apartment that blew up when police raided it, killing three officers, Mr Alamdeen said.
(Except for the headline, this story, originally published by dailymail.co.uk has not been edited by SLM staff)