It remains one of the UK’s most shocking gangland killings. In the early hours of 2 January 2003, best friends Letisha Shakespeare, 17, and Charlene Ellis, 18, were gunned down as they stood outside a new year party in Aston, Birmingham.

Mistakenly killed in an escalating feud between rival inner-city factions the Burger Bar Boys and the Johnson Crew, their double murder propelled both gangs to national notoriety.

Now an investigation reveals that nearly two decades after the conviction of those responsible for the killings, individuals allied to the Burger Bar Boys continue to target the families of the dead teenagers.

Neil Woods, the undercover police officer who infiltrated the Burger Bar Boys by pretending to be a heroin addict. Photograph: Adrian Sherratt/The Guardian

Sophie Ellis – the twin sister of Charlene – says she still faces hostility and intimidation from some of the local community, a continuation of a campaign of violence that she says erupted after the 2005 conviction of four men for the murders.

Sophie, who was shot and severely injured in the original attack, tells a BBC podcast released this week: “I remember my car being smashed. Windows being smashed. We had to press the panic alarm for the police to come, because obviously they knew where we lived at the time.

“And then there’s been times when I’ve been in church and people are making comments like: ‘Oh, that’s the girl that got my people’.”

Dave Mirfield, the West Midlands police officer in charge of the murder investigation, confirms that intimidation by the Burger Bar Boys was very real. He tells the BBC Radio 5 Live podcast: “I do get the fear, and I understand when everybody knows where you live. One of the mums had her windows shot out and that is the intimidation that comes with it.”

Named after two cafes in Handsworth where black youths met in the late 80s, the Burgers and Johnnies formed as vigilante-style gangs to fight far-right groups who were targeting areas of inner-city Birmingham.

As the rightwing extremist threat receded in the 1990s, they turned to the lucrative trade in crack cocaine and heroin, becoming infamous locally for severe violence and kidnappings.

Sophie, the daughter of a founding member of the Johnson Crew, Arthur ‘Super D’ Ellis, describes the aftermath of being shot with a machine gun as she stood outside an Aston hair salon.

“I had to learn to walk again because I was in bed for so long. I still suffer with those injuries today. Some of them are lifelong injuries. I am grateful that I’m still here, but nothing can replace the fact that my twin sister is not here. “

As the influence of the two gangs grew, court cases against the Burgers began to collapse amid allegations of witness intimidation. In response, undercover police officer Neil Woods infiltrated the Burger Bar Boys by pretending to be a heroin addict after the gang expanded to Northampton following the 2003 shootings.

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After weeks of attempting to get close to the gang’s main players, Woods met them in the men’s toilet of a snooker club where the gang had based themselves. He tells the podcast how, trapped in a cubicle, he feared for his life after being cornered and interrogated by four hooded figures.

“Every so often, one of these figures would hit me on the side of the head. I still remember it as feeling like I’d been head butted on the ear.

It came to a point where I thought: ‘Well, I’m not getting out of here in one piece.’ I knew the reputation of them. I’d seen the intelligence, heard the way that people were talking about them on the streets. They left people in a bloody mess all the time.”

Woods was most shocked at how the gang used sexual violence to settle scores, confirming the Burgers were the “most scary” gang he investigated in his career. “I hadn’t really come across that before. They were sexually abusing women and men; they were gang raping for a drug debt.”

Although many members have been jailed since the 2003 murders, the Burger Bar Boys still exist. Last year six men linked to the Burgers and Johnnies faced banning orders after clashing in a Birmingham bar.

Two months ago detectives probing the fatal daylight shooting of a suspected Burger member issued a new appeal to find two brothers wanted for murder.