It was staff at Bunnings who alerted police after a young man made an unusual purchase in December 2016.

He bought 700 nail gun cartridges packed with gunpowder, that were kept behind a locked counter, and left the hardware superstore without buying anything else.

Staff were so suspicious that one of them trailed the man out into the Broadmeadows store carpark on Pearcedale Parade to take down the registration details of his sedan and raise the alarm.

It turned out federal police were already watching his every move as they began unravelling what could have been one of the most deadly homegrown terror attacks on Australian shores.

PAUL JEFFERS/THE AGE Ahmed Mohamed's purchases alerted staff at hardware store Bunnings.

The buyer was Ahmed Mohamed. Then aged in his early 20s, he had grown up in Melbourne's outer suburbs and was living a normal suburban life in Hallam with his wife and a baby boy on the way.

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But underneath that veneer of normality, Mohamed and three of his friends were living a secret double life.

On November 2, a Supreme Court jury found Mohamed, 26, Abdullah Chaarani, 28, and Chaarani's cousin, 23-year-old Hamza Abbas, guilty of planning to carry out a mass bombing attack which was also to have featured knives and guns on or around Christmas Day 2016 in Melbourne's CBD.

GETTY IMAGES The men went to Federation Square to scope it out for a terror attack.

In 2016 the trio were recruited by Hamza Abbas's brother Ibrahim Abbas, who'd spent two years radicalising himself with Islamic State and al Qaeda propaganda, and had already been raided by ASIO officers years earlier.

All four men were born in Australia to migrant parents, were educated, and employed in various jobs such as painting, electrical work and washing cars.

While little was revealed in court about Mohamed's home environment, the Abbas brothers and their cousin had lives plagued with strained relationships and broken homes.

Hamza Abbas' lawyer described her client as having a "fish brain" as he struggled to retain information and concentrate on tasks other than computer games. He was referred to as a tag-along loner who did not want a wife, instead choosing to live with his mother.

Secretly, they'd begun to follow an extreme brand of Sunni Islam which had as one of its central aims the waging of violent jihad against its perceived enemies around the world.

Through the far reaches of the internet, the group connected with Islamic State militants and watched videos of former al Qaeda leader Osama Bin Laden as they preached hate and filmed bloody murder.

PAUL JEFFERS/THE AGE Abdullah Chaarani arriving at the Supreme Court, Melbourne.

They'd meet in places such as the Hume Islamic Youth Centre (HIYC), a mosque complex in Coolaroo, and had begun to follow the same bomb-making manual used in the deadly Boston Marathon terror attack in 2013.

When two bombs went off near the finish line of the US marathon in April of that year, three were killed and more than 260 wounded.

Ibrahim Abbas told the court he was motivated to become a martyr as it offered him a fast-track ticket to "paradise" – a perceived Islamic heaven he said other Muslims only reached when the world ended.

Mohamed and Ibrahim Abbas were also prepared to use their own family members as unwitting weapons and were secretly recorded by police discussing strapping their siblings and wives into suicide vests as they fine-tuned plans for their Islamic State-inspired attack.

"The bigger, the more terror is achieved, and that's the point," Ibrahim Abbas told the court.

PAUL JEFFERS/THE AGE Ibrahim Abbas, who'd spent two years radicalising himself with Islamic State and al Qaeda propaganda, recruited the trio.

But despite Ibrahim Abbas' efforts to conceal his plans from those outside the group, he began to slip up.

It was then police started to listen in.

SHOPPING FOR A TERROR ATTACK

Operation Kastelholm soon learnt the trips to Bunnings in November 2016 enabled the purchase of galvanised pipes, and nail gun cartridges.

A month later two machetes were bought from a BCF store in Coburg and, on December 2 that year, the group travelled to Chemist Warehouse in Campbellfield to purchase hydrogen peroxide, an ingredient suggested in an online video on how to make a bomb "in your mum's kitchen".

On December 20 the four men travelled to Federation Square under the guise of buying ice cream. Instead police said they scoped out potential locations for a terrorist attack.

"I was forcing them that we should wear vests, explosive vests and then, um, we're gunna ram a policeman and get his gun and then I was gunna give that gun to whoever I deemed fit to use the gun," Ibrahim Abbas later told police.

"Then we were gunna go to the city square and, um, one person would use the gun and I was gunna just – whoever I see I was gunna chop and chopping to kill.

"And then I was gunna tell … my co's to blow themselves up."

GETTY IMAGES Concrete blocks, as seen on Flinders street, were installed around Melbourne in 2017 to block vehicle access to pedestrian zones so as to prevent a London or Nice inspired terror attack.

For months the men plotted and planned the ways they could kill innocent people, unsuccessfully trying to build a homemade bomb, before their dramatic roadside arrest at one of the biggest intersections in suburban Melbourne.

At the corner of Dandenong and Springvale roads on December 22, 2016, guns were drawn as Victoria Police's Special Operations Group demanded Mohamed and Chaarani crawl on hands and knees from a red sedan.

Their reign of imminent terror had finally ceased.

"Go ahead martyr me. I welcome death," Chaarani chanted.

"You [police] will have your day soon. Praise Allah, you can't stop us all."

Mohamed, Chaarani and Hamza Abbas are due to be sentenced in coming weeks.