As the sun rose over the Hyde Park neighbourhood in Austin on that Tuesday, a young man and woman sat on the sidewalk outside his apartment complex, savouring the last precious minutes of a date.
Moen, who was then 17, watched intently from behind some bushes. As TV station KEYE noted, when the man ran upstairs to use the bathroom about 7.30 am, the woman began to doze off in the morning light.
That's when Moen struck, bursting from the bushes and brandishing a knife. She stabbed the woman 21 times and left her there to bleed.
As the victim's father told KXAN, "She kind of looked up, closed her eyes, and then all of the sudden she opens her eyes to someone stabbing her repeatedly."
Moen described the attack in further detail in her diary, explaining excitedly how the victim fought back.
So, okay, I'll start with the exciting bit. I stabbed an innocent woman to death earlier today. (Technically yesterday since it's 1 am). It was absolutely fantastic. Murder gives me a high unlike any other. It feels like this crisp unreality, flashing and sparkling, adrenaline and shock. Fight or flight mode. How do I even go about describing it. The whole thing was unreal. I'm so proud of myself. I stabbed her like 20 times. Maybe more. I wasn't counting. She screamed & grabbed at me saying "what the f-?! Help. Leave."
The diary entry continued, explaining she did it because "I'm a homicidal psychopath. I have a deep hatred towards people right now."
Only Moen miscalculated, because the victim, an unnamed nurse, was still alive - if only barely - in a pool of blood on the pavement. She had a collapsed lung, severe tendon and nerve damage, and clipped muscles, KEYE reported.
But she was alive.
The victim was rushed to a local ICU, where over several months she underwent various surgeries, many blood transfusions and demanding bouts of physical therapy, according to the Austin American-Statesman.
"Recovery is a slow road," she told KXAN. "I had some of my muscles cut, so my arm is immobilised for the time being."
Months later, no arrest had been made in her stabbing. Scars, both physical and emotional, remained.
"She cannot understand how someone could do this, not only to her, but another human being," her father told KXAN. "I cannot get my head around it, and neither can she."
Neither could the victim's neighbours, who mobilised to assist police in finding the perpetrator by posting highlighter-yellow fliers around the neighbourhood with what few details were known about the attack.
About four months after the assault, the fliers finally worked.
On Valentine's Day 2016, police were summoned to Moen's house by a family disturbance call. During that visit, her mother told police that not only did Moen fit the description on the ubiquitous posters, she also had a collection of knives and had made jokes about the stabbing.
During the ensuing investigation, police found Moen's journal along with several hand-drawn pictures that appeared to depict the stabbing, CBS reported.
In addition to describing the attack in her journal, Moen also wrote, "I lost my other gold ring I've worn all my life on a chain as it was ripped off by a girl I was murdering. Fate is weird."
Police had recovered such a ring from the scene of the stabbing.
On February 28, 2016, Moen was booked at the Travis County Jail on a $1 million bond and remained there until Friday's plea and sentencing.
And with her arrest, the victim has found at least some peace.
While the victim's identity remains withheld, KXAN published a filmed interview with her.
In it, her shirt hung off one shoulder, displaying a fading pink scar. She pulled up one of her sleeves and said, "These are going to be lifelong cuts." Wrapped around her arms were thick, ropy, raised scars - ones that will not fade.
"I'm happy she's behind bars," the victim said, referring to Moen, "and she can't do this to anyone else."
