'My son was jailed for a murder he didn't commit

Wednesday 20 October 2004 had been an ordinary day for Wendy Cohen. Around three o'clock, she went to pick up her seven-year-old daughter, Daisy, from the school near their neat maisonette on the Arden Estate in Hoxton, London. Wendy's two oldest boys were out working. Upstairs, she had left her other son, 18-year-old Sam. He had recently finished secondary school and was helping out his kitchen-fitter father – from whom Cohen is separated – while applying to join the Army.

"When I got back," Cohen recalls, "there were all these policemen. They were searching everywhere – my room, Sam's room, even taking washing out of the machine." As she speaks, her eyes fill up. "One of them said, 'We need to talk to you, it's serious, but not in front of your daughter.'" Then they told me that they believed Sam had murdered someone. I couldn't stop shaking. I felt physically sick."

Sam Hallam was held on remand at Feltham Young Offenders' Institution in west London until his trial, along with eight others, in September 2005, for the murder of Essayas Kassahun, an Ethiopian trainee chef. Kassahun had been stabbed during a clash between groups of youths from Finsbury and Hoxton. Though no one had reported Hallam as present at the scene, two of the original witnesses subsequently came forward to amend their evidence and name him as part of the group that had attacked Kassahun. For officers from the Metropolitan Police's Specialist Crime Directorate dealing with gang violence, this was sufficient to charge Hallam.

Of the defendants, only two were found guilty of murder. One was Hallam. He received a mandatory life sentence, with a recommendation he serve at least 12 years.

"I'd always believed in the system," says Cohen. "That's how I'd been brought up. But when they brought in the main witnesses against Sam, the first one contradicted his statement to police and then the second one, an old girlfriend of the man who died, said, 'I saw someone who looked like him. If it wasn't him, I saw someone who looked like him.' She then added, 'I was just looking for someone to blame on the spot really.' When I heard that said in court, I thought: 'That's it, there's no reliable evidence against Sam. He's got to be coming home.'" But he wasn't.

In March 2007, there was an appeal. The appeal court judges acknowledged "substantial inconsistencies and difficulties in the prosecution case", but decided that the evidence of the second witness placing Hallam at the scene "was capable of supporting a conviction".

The verdict then rested on Hallam's word against the girl's. His case would be helped hugely if he had a strong alibi. He was, he says, out playing football with a friend. But the friend – despite initially reassuring a member of the Cohen family that he could back Sam's alibi – told police he was working that night. Later, though, he said he hadn't been working and in court said he could have been mistaken about the date when he played football with Hallam.

That lack of clarity eats away at Cohen. What has sustained her has been the extraordinary support for the Sam Hallam Campaign, with high-profile supporters including the actor Ray Winstone – who presented a Tonight programme about the case on ITV last September. On the Arden Estate, every window displays a "Justice for Sam Hallam" poster. That keeps Cohen going. That, plus "the knowledge that when Sam puts his head down at night in prison, he knows he does so as an innocent man."

More details from www.samhallam.com