A series of "coded" handwritten notes made by Lucy Letby were crucial in leading to her conviction.
Police said the mass baby killer used "coloured asterisks" to mark significant events.
The nurse from Hereford has been sentenced to a whole-life order - the most severe punishment available in the UK criminal justice system - after being convicted of murdering seven babies and attempting to murder six others while working in the neonatal unit at the Countess of Chester Hospital.
Mr Justice Goss told the 33-year-old there was "premeditation, calculation and cunning in your actions" as he sentenced her at Manchester Crown Court on Monday.
During Letby's 10-month trial at Manchester Crown Court, the jury was shown a number of notes she had written which offered a glimpse into her state of mind at the time of the killings.
The notes were discovered during police searches of her three-bedroom home in Westbourne Road, Chester.
The prosecution said Letby effectively admitted her crimes in the notes found at her home in July 2018 after her arrest.
In a "documentary" Cheshire Police produced about its own investigation (it was criticised by commentators on BBC Radio 4's Media Show this week as the force's self-production did not allow for external scrutiny), Det Insp Rob Woods said: "The amount of material we found at her home address was a massive surprise to us when she was first arrested, but it gave us a good steer on the second occasion as to what sort of things we were looking for.
"As an example, something that has proved very useful to the inquiry has been Ms Letby’s diaries. There appeared to be - and it became clear later - there was almost a code of coloured asterisks and various other things put in her diary that marked significant events in our investigation.
"So when we went to search the address for the second occasion, that was something we knew we were looking for because we didn’t have the complete chronology."
Det Insp Woods added: "We also knew she was a copious writer of notes. We thought perhaps, having been arrested, she might stop doing that. It turns out when we searched her second address she continued to write her thoughts and all sorts of processes about the investigation, about the events she was being investigated for."
'I AM EVIL I DID THIS'
At the beginning of her trial, in October 2022, the jury was shown a green Post-it note on TV screens by the prosecution.
Letby had written on the note, in capital letters, "I AM EVIL I DID THIS”.
The note was found in a police search of her three-bedroom home on Westbourne Road, Chester, after she was arrested in July 2018.
It was found inside a 2016 diary that was recovered from a chest of drawers.
Paperwork relating to many of the children who died or suffered collapses was found along with Post-it notes, with closely written words which included the names of some of her colleagues, the court heard.
Letby had also written on the green Post-it note: "I don’t deserve to live. I killed them on purpose because I’m not good enough to care for them. I am a horrible evil person.”
This note was referred to several times during the trial.
On 20 June, prosecutor Nick Johnson KC said the note was "literally a confession'.
On 9 June, he told Letby in court: “You felt like this because you knew you had killed and grievously injured these children.”
'I don't know if I killed them, maybe I did'
The court heard that in another note, an A4-size piece of paper found in the diary, Letby had written: “I don’t know if I killed them, maybe I did, maybe that’s all down to me.”
She had also written on one yellow Post-it note, "Love was all we needed", a lyric by the singer Craig David.
On 2 May, the court heard that Letby had written in notes found in her home that two consultant colleagues were "b******s".
They were among senior consultants who had told hospital bosses they wanted Letby removed from the unit because of the "accumulating" number of deaths and collapses of babies.
'I can't do it any more'
A series of handwritten notes found at Letby's home were shown to jurors on 17 April.
Two phrases repeatedly used in the notes were “Please help me” and “I can’t do it any more”, the court heard.
One note read: “No one will ever understand or appreciate what it’s like”, while another said: “I want someone to help me but they can’t so what’s the point in asking. Hate my life.”
In one of the notes, Letby made a declaration of love for a doctor who cannot be named for legal reasons.
Next to his name was written: “I loved you” and “My best friend”.
Letby cried in the dock as photos of her bedroom, taken by a crime scene photographer, were shown in court.
Three handwritten notes were recovered by police from one of the handbags in her room.
In the 2016 diary that was recovered from a chest of drawers in Letby's bedroom, a reference to “twins” was recorded for 8 April.
This was the date of birth for two twin boys prosecutors said Letby tried to murder.
Further entries on 23, 24 and 25 June included initial letters which the prosecution said related to three other babies, including two triplet boys, that Letby allegedly attacked on those dates.
'Everything had got on top of me'
Letby claimed she wrote the notes at a time when she feared her practices may have been at fault for the babies collapsing and when she was "isolated" from colleagues after being moved from the neonatal unit to clerical duties.
In excerpts from police interviews after her arrest, that were read to the jury, Letby said: “I just wrote it because everything had got on top of me.
“It was when I’d not long found out I’d been removed from the unit and they were telling me my practice might be wrong, that I needed to read all my competencies – my practice might not have been good enough.
“I was blaming myself but not because I’d done something (but) because of the way people were making me feel."
Her defence barrister, Ben Myers KC, told the court: “You have seen the notes. They are full of distress, self-recrimination and anguish.
“They certainly do show a very distressed woman. Someone in a terrible state of anguish.”
But the prosecution said they were confessions of guilt and not the “anguished outpourings of a woman in fear and despair”.