Josh Hanson’s mother and sister sit in the living room of a house in north-west London. From a triptych canvas above the fireplace, Josh looks out across the room. His mother, Tracey Hanson, and sister, Brooke, recall him as a young boy. “He was really lively, just really inquisitive,” Tracey says. “He just wanted to do as much as he possibly could.” This lust for life would stay with Josh until his murder at the age of 21.

Josh was killed in a seemingly unprovoked attack at the RE bar in Eastcote, west London, in the early hours of 11 October 2015. Standing at the bar, Josh was stabbed in the neck in front of about 40 horrified onlookers. His killer quickly fled the scene as Josh lay dying on the floor.

Tracey Hanson with Josh as a young boy and his sister Brooke. Photograph: Tracey Hanson

Scotland Yard detectives have no idea why Josh was murdered, but they do have a prime suspect: Shane O’Brien, 27, from the Sutton estate in Ladbroke Grove, west London, who is described as white, 6ft tall and of muscular build. He is the father of two children and has the name Shannon tattooed across his back.

Shane O’Brien, who is being sought by detectives investigating the murder of Josh Hanson. Photograph: PA

O’Brien’s face has become all too familiar to Josh’s mother. As she battles the demons of grief, she has turned the hunt for him into a near full-time job, printing off thousands of copies of a wanted poster featuring an old mugshot of O’Brien. The posters have been pinned up all over the world – “from Thailand to Twickenham” – with friends, family and strangers assisting the effort. “We’re talking about thousands of posters,” Tracey says. “It’s really hard looking at somebody who the police would like to speak to. The police have assured us that the hard work we’ve been carrying out, they’re getting good responses. We won’t stop doing that.”

Josh’s bedroom, overlooking the back garden, has not been touched since his death, although Tracey has taken to sleeping in his bed to feel close to her lost son. A row of squeaky-clean Nike trainers are lined up along one wall, their boxes neatly stacked to one side. An unblemished New York Yankees baseball cap hangs on a bedpost. Photographs of Josh, his friends and family are stuck to the wall at the foot of his double bed.

Josh was a streets planner for Stanmore Quality Surfacing, a company he joined at 18 after graduating from Watford college with a BTec in sports science. He quickly climbed the ranks and at the time of his death was overseeing paving and tarmac jobs conducted by 600 men. He would leave home at 5am every day and often worked half days at the weekend. He had even started learning Romanian to help him communicate with some of the road workers.

On the night of his death, Josh had accompanied his girlfriend Lucie Carpenter to hospital after a close relative of hers had fallen ill. It had been a long evening and after midnight, following several hours at the hospital, the pair decided to join Josh’s cousin and best friend, Reece Kennedy, for a few drinks at the RE bar. Spirits were high: a DJ was playing R&B music, friends were dancing. Josh went to buy a drink. “I turned my back and this is when it happened,” Reece says. “Someone grabbed me and said ‘Josh, Josh’. I turned round and saw him.” The chaos that unfolded is a blur to Reece now. Josh was still up and moving after the stabbing, despite his injury. Reece remembers rushing to his cousin and holding him. “He was alive for a bit,” he says. “I was talking to him, telling him everything was going to be all right. I didn’t want to leave him. The chances of me leaving him and him dying alone were too high.”

Josh’s last moments were captured in graphic detail on CCTV from within the RE Bar.

Tracey was home alone when she received a call at 1.20am and rushed to the bar, but when she arrived police officers were carrying out forensic tests and so were unable to let her in. “I just wanted to touch him while he was still warm because I knew the next time I saw him he wouldn’t be,” she says.

Josh’s body lay in the bar until 5.30pm the following day. “You get there and think no, this can’t be right,” Tracey says. Two days later, Josh’s cause of death was given: haemorrhage, inhalation of blood and an incised wound to the neck.

Josh Hanson. Photograph: Rex Shutterstock

More than 2,000 people attended a celebration of Josh’s life at Sacred Heart church in Kilburn, from friends and family to former teachers and workers from Nando’s, where he would stock up on chicken and rice before going to the gym. Some of Josh’s favourite music was played: Maria Maria by Santana; Baby Let Me Love You for Tonight by Kariya; Lay Me Down by Sam Smith. Reece, a plumber, and Josh went out most weekends. “We worked five days a week, you like to go out, have a good time,” Reece says. “This time we went out and Josh didn’t come home.”

For many people a funeral or a celebration of a loved one’s life can provide a sense of closure . Not so for Josh’s family. Tracey says she suffers from attacks of guilt and anger, as well as the inevitable deep and heavy sadness within her. “You have the craziest thoughts,” she says. “Like if we had been there we could have stopped it.” Scotland Yard – and Tracey Hanson – say they need the public’s help to find Shane O’Brien. “The public, as with any investigation, are our eyes and ears,” says DCI Noel McHugh, who is leading the investigation into Josh’s murder. “People will have seen and heard things out there about where O’Brien is and someone may be contemplating whether they should make that call.”

O’Brien has links to Ruislip, also in west London, and to Ireland and Spain, including Ibiza. The Guardian can reveal he was seen on 12 October in Camber Sands in East Sussex and then on 13 October in Ashford International shopping centre in Kent, buying clothes. After that, the trail runs cold. “Wherever he is, or whoever he is with, they’re doing a damn good job of hiding him,” Tracey says.

A reward of £10,000 has been offered for information leading to O’Brien’s arrest and prosecution. But it will not be easy, especially when O’Brien has friends or family allegedly helping him avoid detection; two people have been arrested and are on bail on suspicion of assisting an offender. “There’s been a small group of people who have and continue to support Shane O’Brien,” McHugh says. “We’re totally focused on them.”

McHugh says people go to great lengths not to be found. “But we are relentless in our efforts to capture people,” he adds. “Ideally we’d want them caught on the night. However long it takes we’re not going to give up.”

As another day passes with no justice for Josh, the pain felt by his family grows. His protective older sister says the future scares her. “We had so many plans,” Brooke says. “My heart breaks for Josh, because he’s never going to be able to do them.” Plans including music festivals in Austrian ski resorts, a close friend’s stag do, a mortgage application for his own place and most poignantly, a pledge to walk Brooke down the aisle if she got married.

And in the middle of this storm stands Tracey, a heartbroken mother resolved to find her son’s killer before she can really allow the grieving process to begin. “Since Josh is no longer here, Brooke and I have had to readjust together,” she says, gazing at her son’s image above the fireplace. “It was always three. I used to call us the triangle. Not any more.”

