Who murdered the Miyazawa family? Was it a disgruntled skater? A contract killer? For a grandmother and a retired police chief, there is no closure without answers.

The end of the year is one of the most momentous occasions in Japan, a chance to celebrate a fresh start and welcome new beginnings.

It was a frosty winter's night in Setagaya, Tokyo when eight-year-old Niina, her younger brother Rei and their parents Mikio and Yasuko were at home, preparing for the festivities.

But the Miyazawa family would never celebrate that day.

All four were brutally murdered on December 30, 2000 in a crime that shocked Japan and began a grim new chapter in the country's criminal history.

Police know so much about the killer — everything except who he is and why he committed such a violent crime.

He left his DNA everywhere.

He left clothes at the scene.

He left the murder weapon.

He used the victims' computer.

He ate at least four ice creams from their freezer.

And he stayed inside the house for hours as the family lay dead.

Almost 250,000 investigators have worked on this case, receiving more than 15,000 tip-offs from the public.

But nearly two decades later, officers are seemingly no closer to solving a crime that is extremely rare in this largely peaceful country.

'Why would they kill the children?'

Niina was a clever and active young girl.

The eight-year-old was in second grade at school and a year ahead in her studies.

Her grandmother Setsuko fondly remembers how much she loved her and ballet.

"Niina loved to show me her moves. She was just a bright and adorable child," she said.

Niina's six-year-old brother Rei lived with a mental disability, and his parents surrounded him with love and support.

Their father, Mikio, worked for a marketing company and his wife Yasuko was a teacher.

Police believe on the day of the murders, they had been out shopping and spent a relaxing night at home eating dinner and watching television together.

But what happened next still confounds detectives to this day.

The family lived next door to the children's maternal grandmother, Haruko.

The next morning, she was unable to reach the family by telephone as she usually did because it had been disconnected.

That was the first sign something was amiss.

So she went over and knocked on their door.

Again, nothing.

When she opened the door, she found her son-in-law, Mikio slumped at the bottom of the staircase near the front door.

His lifeless body had been stabbed multiple times.

Haruko went upstairs where she found her daughter and granddaughter violently and repeatedly stabbed with a sashimi knife.

She touched them, desperately searching for any signs of life.

Police believe the attacker continued to stab Niina and Yasuko far beyond the point at which they had died.

In Rei's bedroom just around the corner, Haruko found the six-year-old still in his bed.

He had been strangled.

Police believe he was the first to have been killed, spared knowledge of the brutal fate his sister and parents would suffer.

Haruko called police, stained with the blood of her family.

When the children's paternal grandmother Setsuko found out what had happened, she said she blanked out with shock.

Inside her home she keeps a shrine to the family and still prays for them.

She has all of their toys on display in a cabinet.

"I always wonder how they would have grown up. My biggest regret is that I never got to see them grow up," she said.

She still cannot remember the family's funeral.

She was told she was so traumatised that she could not walk and had to be carried inside.

"Why would they kill the children as well? If someone held grudges, they could just kill the adults," she said.

"I just don't understand why, I really don't."

The detective haunted by the faces of the dead

It's a question still seared in the mind of 72-year-old Takeshi Tsuchida.

The now-retired police chief played an integral role in the case — and cannot let it go.

An officer for 41 years, he rose to the ranks of chief officer at the Seijyo Police station, the force tasked with investigating the family's murder.

The facial expressions from the bodies he inspected still remain seared into his memory.

"When you compare victims who die from illness or natural causes to those who are suddenly murdered, they look very different," he said.

"They have furious facial expressions. They are mortified and regretful. I imagine that all of the victims felt the same way, just feeling regret."

Even though he has been retired for 11 years, he is still unofficially working the Setagaya family murder case.

"When I think about the feelings of the victims' families, who lost their loved ones all of a sudden, I just couldn't pass on this case to my successors after working as an officer for 41 years," he said.

"What could I do as a civilian? I could no longer carry out the investigation or put handcuffs on the criminal, but I had a lot of knowledge about this case. So I made my own fliers which ask for information."

He visits the scene regularly to stay familiar with the details.

But there are some details he will not — and cannot — ever forget.

"When I think about the brutality in the way he murdered the four, I just wonder, how could a sane person carry out such an extreme crime?" he said.

"He slashed them from above the chest to the face as if he tormented them. It was extremely brutal.

"And the way he finished them off in the very end … [it was so horrific] we couldn't show those scars to the devastated victims' families. There are no other cases in which the victims have been cut up like this," he said.

Police have many clues about the killer's identity and what happened that night.

Some officers believe the killer climbed up a tree, removed a screen and then entered the open window of the second floor bathroom.

But the exact point of entry still is not clear.

Their home backed on to Soshigaya Park, giving the killer several possible points of entry.

They have the killer's fingerprints and DNA, which so far have not recorded any hits in Tokyo police databases.

They know that the sweatshirt he left was only manufactured and sold 130 times, but officers have only ever been able to track down 12 owners.

They have his blood, which revealed the killer was male and potentially mixed-race, most likely of Korean or Chinese heritage.

They know through his footprints that his shoes were probably made in Korea, in a size that was never sold in Japan.

They believe he was slim, because the bum bag he left behind indicated his waist was between 70 and 75 centimetres.

They even know that he ate string beans and sesame seeds the day before because he left faeces in the bathroom.

"It's been 19 years and despite so many clues left behind, the fingerprints and the DNA of the criminal, why can't we find him?," Takeshi Tsuchida wondered.

For the former police chief, there too many questions left unanswered.

"Why did he enter a room that was lit? We look at each action and think, why? Was it for money? Did he have grudge against somebody in the Miyazawa family? Or was it about something else?"

How do you catch a faceless killer?

Because the investigation cannot work out the motivation of the killer, police cannot narrow their focus.

One avenue of investigation looked at whether Mikio had been seen arguing with skaters at the nearby park in the lead up to his murder.

The killer's clothes and suspected age could suggest a disgruntled skater tired of the neighbour's noise complaints who took matters into his own hands.

But there are other, more outlandish theories.

Investigative journalist Fumiya Ichihashi has spent years researching the case and wrote a book concluding the killer was a former South Korean army soldier-turned-killer for hire.

He believes one of the killer's motivating factors was an attempt to steal compensation money paid to the family over the expansion of the nearby park.

"As the criminal left behind his knives, clothes, bag, fingerprints, palm prints and foot prints without care, I couldn't help but think that he was confident he would not get caught," Mr Ichihashi said.

"My interpretation was that the criminal was not Japanese, did not live in Japan, and he immediately escaped overseas."

He believes police officers failed in their initial investigation.

"When the incident happened, the special investigators at the Tokyo Metropolitan Police were all working on different cases and it had no choice but to send their reserve team," he said.

"Also as it was New Year's Eve, many detectives were at home and it took time to send the investigators. We can't deny the possibility that this led to an unresolved case."

Grandmother Haruko, who found the victims, initially told police she had unlocked the door using her spare key, but in her later years was not so sure.

This has made it much harder to confirm exactly how the killer entered and exited.

Mr Ichihashi said because police could not confirm basic details — no matter how good detectives may be — they would not be able to arrest the criminal.

"Unless there's a miracle and the criminal surrenders himself or his fingerprints match if he commits another crime, I believe there is no chance he will be arrested," he said.

"Japanese police are not great with international cooperative investigations because of a lack of experience and language skills."

Takeshi Tsuchida disputes the author's take.

"It's 100 per cent nonsense," Mr Tsuchida said.

"If the book was true, the criminal would have been caught. It's OK if it was a fictional novel, but I have strong doubts calling it non-fiction."

The next generation takes up the case

The case is now in the hands of a detective decades younger than Takeshi Tsuchida.

Manabu Ide, from the Tokyo Metropolitan Police Special Investigative Task Force, remains confident the case will ultimately be solved.

"I don't think there is any detective who is not confident," he said.

"It's our mission to arrest the criminal who murdered four innocent people, including two young children, and make him atone for his crime."

Superintendent Ide said investigators would like to help the victims rest in peace.

"It's an atrocious case rarely seen in Japan's criminal history and we think solving this case will help prevent similar crimes happening in the future," he said.

The investigators who first worked on the case have now retired and the family fears time is running out.

Police now want to demolish the house which has been preserved for 19 years because it is so old and at risk of collapse.

Superintendent Ide said they had preserved all of the possible evidence inside, so there would be no impact on the investigation if the home was torn down.

"Can the new people who haven't experienced the graphic crime scene themselves focus on the investigation? I feel worried," former police chief Tsuchida said.

Still, Takeshi Tsuchida believes because they have the DNA, there is always hope of a breakthrough.

He stays in close contact with Setsuko, Mikio's mother.

On a recent visit to Setsuko's home, he brought a meal he had prepared for her.

He said hello, then immediately knelt down at the family shrine, lit incense and prayed.

For Setsuko she desperately hopes for answers while she is alive.

"When my husband died the case was still unresolved, and that troubled us the most," she said.

"Now that he's gone, I feel I have to work hard by myself."

Photos of the Miyazawa family hang in her room and she waits for the day she can tell them the criminal is caught.