A teenager killed a nurse after stabbing her more than 70 times because he was "bored", the Old Bailey heard.
Cheryl Moss was found stabbed by her colleagues
Stuart Harling ambushed Cheryl Moss, 33, as she took a cigarette break near woods behind St George's hospital, Hornchurch, Essex, in April, last year.
Jurors heard a statement from a prison guard alleging that Mr Harling admitted killing Mrs Moss because he was bored and could do it again.
The 19-year-old denies murder, claiming diminished responsibility.
But the prosecution claim he was living out a violent fantasy which evolved after he researched murder and serial killers on the internet.
Belmarsh prison officer Christopher Potter was guarding Harling during a suicide watch in August, last year, after Harling was seen making a noose from a sheet.
In the police statement, Mr Potter said: "On the day he murdered her, he was just bored. He could not rule out doing it again."
Mr Potter said he sensed Harling had wanted to speak after watching CSI on television.
He went to bed but got up again. "I got the impression he wanted to talk," Mr Potter told police.
"I might have asked him why he did it. He said he was just bored. He went on to explain he got bored very easily."
The trial continues.