Speaking outside the Old Bailey, Jenny Hopkins, of the Crown Prosecution Service, said Thompson put scores of lives in danger by starting the Croydon blaze, including both firefighters and those living nearby.
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America says it has "identified preparations" for an attack similar to that which killed at least 86, including 30 children, in April.
Despite the rain it will still feel muggy with highs of 22C.
The practice of sending mental health patients far away from friends and family for care has become endemic in the NHS, it has been warned.