Joanna Dennehy told the judge as she pleaded guilty to murdering three men , "I've pleaded guilty and that's that".
Dennehy also admitted "preventing the lawful and decent burial" of all three murder victims.
Her shock guilty pleas left the courtroom at the Old Bailey stunned.
The defendant's barrister, Nigel Lickley QC, said: "The course of the arraignment is not one we had anticipated."
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Police carried out a "proactive" operation after receiving reports that other concerts by the band had seen lots of mobile phone thefts.
The child died on her injuries and another 14-year-old girl was injured after the buggy they were travelling in was involved in a car crash.
Cloud thickening from the west throughout the day. Rain in northwest later.