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On 7th July 2005, a slightly hungover Karl Williams found himself jammed onto a Piccadilly Line tube carriage that he wasn't even meant to be on.
There were delays to his usual journey to college, so the 23-year-old took an alternative route - one so crammed that he had to stand the whole way, his bag twisted around a fellow passenger's.
It's would have been impossible to imagine, in that moment, that a complete stranger standing mere feet away would have any lasting impact on his life.
But seconds later, a devastating explosion took place, and Karl remains adamant it was one woman's outstretched hand and gentle reassurances that saved him that day.
Karl is a survivor of one of the 7/7 bombings - four suicide attacks that took place across London 15 years ago, killing 56, injuring 784, and leaving many more with psychological scars.
The attack happened a day after it was announced London would be hosting the 2012 Olympics. Karl was a performing arts student, and having moved to London from Stoke for the "bright lights, big city," he was struck by the excitement rippling through the city - something quickly overshadowed by the harrowing events that followed.
"There was a huge bang, and then just this feeling of rolling along, like when you have a car and your foot is on the clutch," Karl tells Tyla. "I remember this eerie silence that felt like it went on for a really long time, and when the silence stopped, that's when the screaming started.
"It was pitch black, very smoky and there was a strong smell of acid in the air. I think I must have been aware of what happened, but I lost any control over what I was feeling."
It was only when Karl reached to the floor for his iPod and felt debris up to his knees that he realised how bad things were, and the fear kicked in.
"Where the bomb had exploded it caused the tube to twist slightly right where I was, so there was a lot less room above my head," he says. "But where there were so many people before, there was an element of it being a bit more spacious, which was strange.
"Through the smoke, you could see and hear people, and what made me panic was there was this really large, tall guy and he was desperately trying to climb out of the broken glass in the doors.
"The tube and the walls were very close, and seeing him get stuck made me think, 'I need to get out. I need to get out.'"
Despite not being religious, amid the fear and hysteria, Karl started praying to God, promising he'd no longer be gay if he made it out alive.
"It was just complete desperation," he says. "When you're faced with death so closely in that way, and you have this feeling of death all around you... I felt so alone and thought I was going to die."
Karl firmly believes what happened next changed his outlook on the terror attack forever.
"I started crying quite loudly, and this lady just reached over and said, 'Is it ok if I hold your hand? I just want to hold someone's hand'.
"There was a brief moment of feeling a little weird about that, but I was so desperate in that time that I held her hand, and I said to her, 'We're going to die here aren't we?'
"She said 'No, we're going to get out,' and she was so calm and peaceful.
"In that moment, I feel like she saved me. It sounds weird because I wasn't physically harmed, but she gave me hope, and my life would have been and a lot more bitter and resentful if I hadn't have had that experience."
The pair held hands in that carriage for an indeterminate but seemingly never-ending amount of time, and then the tube was evacuated, and Karl entered "get me out of here mode".
"I didn't physically push past her, but I didn't let the lady who held my hand go first, which is very uncharacteristic of me," he says. "To this day it's something that I still feel terrible about."
Karl suffered PTSD in the aftermath of the attack, and while therapy has helped, among other memories from that horrific day, his encounter with the woman is something that has always "haunted" him.
"I understand survivor guilt, and it was what I had to do in that moment, but I'd love the opportunity to say sorry, and thank you," he says.
While he's written in survivors Facebook groups before, and even appeared on the news, the hunt for his carriage companion had never proved fruitful.
However his latest attempt, on BBC's Saved By A Stranger, offered him some emotional relief, even if not all the answers.
The aim of the docu-series - which airs on BBC2, on Thursday 29th April - is to reunite people with strangers who had impacted their lives for the better. And, amazingly, the team did find somebody from Karl's carriage who shared similar memories of that day.
Susan (the woman in question) had held someone's hand while standing in the wreckage. She, too, had taken comfort from that moment of human connection, and she had also lost contact with the man right before they were evacuated, because she knelt down to pick her phone up.
The only problem is, she didn't fit Karl's memory of that 'person' - although he's the first to admit his memories may be hazy.
Whether she is the woman he's looking for, we may never know.
While Karl was left with a seed of doubt, he says he found the interaction cathartic, nonetheless.
"What Susan said was that [holding hands] helped her at that time, and all I've ever done is talk about how selfish I was in that moment," he reflects. "I was never thinking that I might have been a support for someone else."
He adds that "in a sense, it's irrelevant" whether she's the woman or not, explaining: "Hearing her experience was really freeing for me, and it helped to shift that guilt that I had been carrying."
Following 7/7, Karl continued performing arts for around five years, but - driven by a need to understand what happened to him - he eventually went on to study International Relations and Anthropology at the University of Sussex, before undertaking an MSc in Psychology of Mental Health.
"The bombing was such a catalyst of change for me," he says. "If I had been in the same position in any of the other carriages I would have been killed, but because my carriage was so busy and packed, the people who died took the impact, so I felt like I owed them something."
Now, Karl plans to become a clinical psychologist, so he can help others who have had similar experiences - a pursuit he believes he may not have had he strength to undertake without the support of that woman, 15 years earlier.
"So many people lost their lives that day, and even with the people who survived, the bomber took elements of their life away. She somehow helped me to mitigate that. Not to forget it - but just to carry on," he says.
"What's really important is that even in those dark moments there's this beautiful humanity as well."
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