The victim went missing months earlier, his body disappeared long ago and the crime scene was cleaned just like they do in CSI. With every passing day the trail became colder and soon police would have to move the file from the active pile to the pending one.

There were only two men who knew the truth, the killer and his apprentice, but while the senior partner was relaxed after a two-month Mediterranean holiday, the younger was wrestling with his conscience. And the conscience was winning.

On April 5, 2010, he confided to his mother that while he was not the hands-on killer, he was the next worst thing. She told him only he could decide what to do but he must be prepared to live with the consequences.

Aged just 19, he finally decided a possible jail term and a new start would be better than spending the rest of his life hiding the awful truth. So around 1am the next day the apprentice welder walked into the Craigieburn Police Station and announced he wanted to talk to a homicide detective about Peter Rule, a man who had disappeared five months earlier.

Like all murder investigators, Senior Detective Paul Rowe knows interrupted sleep is part of the deal and was soon driving to the Broadmeadows station where the witness had been transported.

Rule, 56, was reported missing on November 17, 2009, by his Meadow Heights neighbour. He was separated from his wife but always made a bedtime call to his 11-year-old son. But since that date he remained silent, his mobile phone was inactive and his credit cards unused.

Within two weeks the case was handed to Homicide Crew Four - the suspicious disappearance and cold case unit.

Rowe quickly knew this fresh witness nursing a lukewarm coffee and a Subway sandwich was the real deal - because he wasn't trying to make one. After all, this wasn't an arrested crook trying to give up an accomplice for a shorter jail term - he had walked off the street to admit his guilt without inducements.

For 40 minutes without a break he outlined what had happened and named the killer.

Leonard Borg, then 25, was a man whose limited talent didn't thwart his big ambitions. A boilermaker and steel fabricator by trade, he planned to become a major player in the drug trade.

Having spent years working in industrial estates, he saw a lucrative opportunity in turning a disused prefabricated factory into a giant hydroponic cannabis centre. There would be little through traffic, large power bills would not be considered unusual and there would be no nosy neighbours concerned about late-night visitors.

In early 2009 his trial run of just 20 plants in a rented Craigieburn house ended badly.

After hearing whispers that police were closing in, he ripped out the plants and, determined to leave no forensic clues, painted the inside of the house a week before the inevitable raid.

Borg found a cap at the property he believed was Rule's and concluded he was the one who had tipped off police (in fact the information came from a call to Crime Stoppers by an unidentified female).

Borg knew Rule and they shared an interest in vintage motorbikes. When Borg fell out with his father, he moved in with his mate.

However, such an act of kindness was forgotten. Borg had already rented a Thomastown factory for $25,000 that he was turning into an indoor cannabis field and was concerned Rule could dob him in again.

His mate would have to go.

For a first-time killer, Borg was methodical, cunning and cold. First he needed a gun and drove to northern New South Wales where he bought a semi-automatic .22 rifle from a farmer. A week later, on Sunday, November 15, he invited Rule out on the pretence of finding a vintage motorcycle, shouted him a kebab then took him to a friend's Campbellfield factory, using a key hidden under a rock to gain access.

Once inside, Borg started a truck in the factory to conceal the noise, picked up the gun he had hidden behind a wheel and shot Rule 10 times in the head and body.

He later complained he could find only nine spent cartridges.

Next he rang the apprentice who was playing pool in Epping to instruct him to buy bleach and cleaning products at a supermarket before driving to the factory. The two men used 25 litres of bleach to scrub one toilet cubicle, leaving the second in its existing state - a serious mistake as it turned out. Much later, police would see it as an obvious attempt to conceal a crime scene.

Borg used a forklift to place the body in the boot of his blue Volvo and drove to his Thomastown factory. The killer was determined to leave no forensic clues and almost succeeded. But in the murder business, a miss is as good as a mile and he would eventually be undone by the tiniest of oversights.

At a local service station he bought nearly 20 bags of redgum and pine firewood and 15 litres of petrol, before stopping to eat.

He went to Bunnings and bought an electric chainsaw, hatchet, citronella oil, a black tub and hydrochloric acid.

There is no need to go into minute detail except to say all the products he bought were used in the cover-up.

At one point he placed the ashes on the floor of the factory and used a hammer to smash any remaining pieces of bone fragments before placing them in the tub filled with acid. What remained was scooped into garbage bags.

Borg and the apprentice drove down the Great Ocean Road and found a remote beach past Anglesea where they waded into the sea to dump the sluice from the tub. The ashes from the garbage bags were dumped near some foreshore bushes.

But as the acid had burnt holes in their clothes they drove to Lorne to buy a change at Louttit Bay Menswear, choosing two T-shirts and two pairs of red LBAY shorts.

So determined was Borg to leave no clues, he and the apprentice painted the whole floor of the rented Thomastown factory with blue paint. If he had worked as hard at his legitimate job, he wouldn't have turned himself into just another failed gangster.

Four days after the murder Borg headed to Malta for what he thought was a well-earned rest, leaving the apprentice to maintain the cannabis crop. The sidekick, however, overwatered the plants and they all died.

When the younger man finally confessed, Senior Detective Rowe and the rest of Crew Four knew he was telling the truth. But in the murder investigation business a miss is as good as a mile.

The killer had already been nominated as a suspect when one of the victim's friends said Rule told him he was going to meet Potato Head (Lennie Borg's apt nickname) on the night he disappeared.

Later, police would find Borg tried to get a factory owner to fake him a time card for the night of the murder as a back-up alibi.

Their job was to try to corroborate every part of the story. At the Campbellfield factory police found a speck of blood on the toilet door frame and a nearby wooden pallet. So far so good.

The apprentice took them to the beach where they found two tiny bone fragments in the ash pile, remarkably undisturbed from five months of ocean winds and rain. Then they went to the nearby menswear shop to recover a receipt for the change of clothes.

Later they would find a matching pair at Borg's home - and Louttit Bay Menswear was the only Australian distributor for the line. Bingo! They would also find the gun, a cartridge box with 10 bullets missing, the smashed black tub used to take the remains to Anglesea, and the paint tins used to repaint the floor in the Thomastown factory.

Just 18 days after the apprentice decided to come clean, police grabbed Borg and took him to the homicide squad office for an interview. Mr Potato Head lived up to his reputation - as an interviewee he was an absolute spud. When asked if he had ever been to Lorne he answered, ''I can't recall.''

When confronted with overwhelming evidence of his cold-blooded acts he showed no remorse and simply stared, looking like a carp caught in a net.

On the way to the custody centre, when it finally dawned on him there was no one on the outside to look after his two guard dogs, he burst into tears.

Last month Justice Lex Lasry sentenced Borg to 23 years with a minimum of 19 - three years to the day after he headed to Malta on a long holiday convinced he had committed the perfect murder. At the 11-week trial the apprentice gave flawless evidence (he had earlier received a suspended sentence).

''He was brilliant,'' said Detective Senior Sergeant Ron Iddles. ''If he didn't have a conscience we would have never known the truth.''