Killer nurse Lucy Letby used a “coded system” in her diary to record her crimes.
Her jottings formed a crucial part of the case against her, revealed a police chief.
Detective Inspector Rob Woods said Letby was a “copious writer of notes”.
During her trial, dozens of post-it notes and scraps of paper found in her home in Cheshire, were shown in court.
In one, she wrote: “I killed them on purpose because I’m not good enough.”
Letby also chronicled the police investigation into the deaths of the babies.
DI Woods, speaking in a documentary made by Cheshire Constabulary, said:
“The amount of material we found at her home address was, I think, a massive surprise to us when she was first arrested.
“It gave us a really good steer for the second occasion as to what sort of things we were looking for.
“So as an example, something that’s been very useful to the enquiry has been Miss Letby’s diaries.
“They appeared to be and it became clear later that it was almost a code of coloured asterisks and various other things put in a diary that marked significant events.”
Letby, a 33, a neonatal nurse, was sentenced to a whole life order after being convicted of the murders of seven babies.
She was also found guilty of attempting to six more infants while working at the Countess of Chester Hospital.
DI Woods said it became clear that the events Letby had made notes of were significant events connected to the case.
He added: “We knew she was a copious writer of notes.
“Now we thought that having been arrested she might stop doing that.
“It turned out when we searched that second time, she had continued to write her thoughts and all sorts of processes about the investigation about the events that she was being investigated for.”
