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Grace, from Essex, had been on a round-the-world trip when she met a man named Jesse Kempson on Tinder. Kempson was later found guilty of Grace's murder.
Now, in a new documentary, The Murder of Grace Millane - A Faking It Special, a team of experts can be seen analysing chilling footage of Kempson, before pinpointing the early moments he gave away his guilt.
You can watch a clip of the moment below:
In the doc, body language expert Cliff Lansley pointed out the first clue that indicated Kempson's guilt.
He explained: "As soon as the word Grace is mentioned, he starts to tense up. We see this little tweak under the table of the legs coming together, and squeezing his hands in between his thighs.
"This is an indication of 'okay here we go, this is the difficult area, I've now got to speak about Grace'. So if he wants to get away with deception or lies, one of the things he can do is clamp himself with his hands and try and stay still."
Kempson then takes a drink, with Cliff adding: "The tension is reinforced by him taking a sip of water. When we get anxious our mouth goes dry, and often we'll swallow or lick the lips or take a sip of water when we hit an anxiety point.
"... We've got three indicators of anxiety or fear, this could be the fear of being caught in a lie."
The officer then goes on to ask Kempson how his evening with Grace panned out. He takes a three second gap, before saying: "Mm, yeah pretty good."
"We've got a huge hesitation, he's having to think hard about a question that would be simple for a truth teller but difficult for a lie teller," said Cliff.
Meanwhile, linguistics expert Dawn Archer added: "There's a three second pause, which for me is quite odd because it's an easy thing to answer, isn't it.
"If you've had a really nice evening you don't need to pause for three seconds and then say 'mm, pretty good'. His volume drops significantly at that point, so we've had a stress indicator through the voice quality and now we have a distance indicator through the volume drop."
Cliff adds: "He can't give detail about the evening, so the officer is on to him."
Grace was first reported missing by her family in Essex who alerted police when they failed to hear from Grace on her birthday.
Police discovered Kempson had been on a date with Grace just days before and was the last person to message her on Facebook before she went missing.
Kempson denied murdering Grace, instead maintaining she passed away during consensual rough sex.
Grace's body was found a week after her disappearance, buried inside a suitcase.
The court heard how, on the night of her murder, the killer "wasn't distressed or concerned" by her passing, but instead set about planning how to dispose of her.
They were also told how he "sexualised" her killing, taking photographs of her dead body and searching for pornography.
He also went on a Tinder date with another woman while Grace's body sat in a suitcase in his bedroom.
Kempson has been jailed for life and sentenced to a minimum of 17 years.
The Murder of Grace Millane - A Faking It Special is available to stream now, on discovery+.
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