The 16-year-old defendant, known as girl X, claims she had her back to her co-accused, boy Y, who is also 16, and Brianna when she "heard a noise like someone screaming" and turned around.

"I saw boy Y stabbing Brianna," she said.

"It looked like she was starting to fall towards the floor because she was kind of like leaning a bit."

Girl X said she saw boy Y stab Brianna, who was transgender, "around five times", adding: "I seen her fall and land face down on the floor.

"I saw him stand next to her leaning over, looking down on her," telling Manchester Crown Court he "could've" stabbed her again while she was on the floor.

Brianna was stabbed 28 times in her head, neck, chest and back with a hunting knife after being lured to Linear Park in Culcheth, near Warrington, on 11 February, the court has heard.

Girl X and boy Y, who were both 15 at the time and cannot be identified because of their age, deny murder and are blaming each other for the killing.

Girl X became visibly upset at one point while giving evidence today, wiping away a tear when recalling the moments after Brianna's death.

Asked by her barrister Richard Pratt KC how she felt when she saw the boy stabbing Brianna, she said: "I was in shock really when it happened."

She added: "There was nothing really I could do because if I tried to stop him, boy Y could've easily overpowered me."

She claimed she had not expected the alleged attack and had not wanted it to happen, claiming previous discussions about killing Brianna had been a "fantasy".

"I walked over to see if I could see her breathing and I couldn't. I saw boy Y start to run and then I panicked and started running with him," she said.

"I kept thinking about whether I should go back or not."

Girl X denied she had stabbed Brianna, that she wanted boy Y to stab her or that she had wanted the victim to die.

Asked how she feels about Brianna's death, she said: "I do feel really upset about it."

The court heard Girl X watched videos of Brianna and sent messages about the murder to friends in the hours after the stabbing.

She told her mother, and then the police, that Brianna was with her but had left to meet a boy from Manchester.

She told jurors: "I was scared because obviously I knew how bad it would look, just based off text messages, so I was defending myself as well as boy Y at the time."

Girl X has admitted watching internet torture videos on the "dark web" but she denied that she "liked seeing Brianna stabbed".

'I just want her to die really bad'

She has previously told jurors she had "dark fantasies" of killing other children, as well as Brianna, and an interest in serial killers, but had no intention of making any of it reality.

Jurors have been shown a handwritten note of an alleged "murder plan" to kill Brianna, which was found on girl X's bedroom floor.

"I thought if I wrote an actual note and showed it to boy Y he would think I was being serious," she explained.

The court has seen messages in which she told boy Y: "I just want her to die really bad" and "I want to see the pure horror in her face and hear her scream in pain."

In another she said she wanted to "keep part of its flesh and an eyeball. Really want one because they have pretty eyes."

Asked why she was talking in that way, girl X said: "Because I knew some serial killers would do things like that," adding that she didn't mean to refer to Brianna as "it" and had not meant the term "in a nasty way".

Girl X said messages in which she encouraged boy Y to bring a knife were "to add more to the fantasy" of killing Brianna and "so he would think I was serious".

Asked if there was a possibility boy Y would think she was being serious, she said "yes" but said she did not think he would bring a knife "because I knew he never took his knives out with him".

The trial continues.