An ex-public schoolboy who threw fire bombs from a motorway bridge in the hope of killing drivers has been jailed for four and a half years.
Nicholas Elger, 17, was sentenced for two arson charges after two attacks on the M3 in September 2017.
The second closed the motorway for 11 hours, causing an estimated £40m of damage to the economy, a court heard.
Elger, who admitted a range of charges, was also ordered to serve an additional four and a half years on licence.
As well as the M3 attacks, he cost former school Winchester College £52,000 in stolen equipment and damage and had twice demanded payment of £10,000 in the online currency bitcoin for him to stop carrying out further break-ins.
At Winchester Crown Court, Judge Keith Cutler said Elger was "a very bright boy and for some reason he takes to burgling the school, blackmailing the headteacher and making incendiary devices and throwing them off motorway bridges".
Tessa Hingston, prosecuting, said Elger has previously been remanded under psychiatric care and had told medical staff that he intended to kill someone in the motorway arson attacks.
She added he told staff: "I do not regret the incident, I regret not doing them differently and not succeeding in killing."
Robert Morris, defending, said doctors had ruled out personality disorder or psychotic illness and Elger's actions had been caused by "immaturity and being upset by his parents' divorce, leaving his school and support networks".
Judge Cutler, who lifted reporting restrictions on identifying the teenager, said Elger had written in a diary in hospital stating he wanted to kill patients and staff by "strangling and jumping on heads" and had also heard a voice telling him to kill his doctor.
Elger, of Winchester, previously pleaded guilty to two counts of arson recklessly endangering life.
He also admitted two counts of blackmail in which he threatened to burgle the school unless the headmaster paid him about £20,000.
The teenager pleaded guilty to eight burglaries, mainly of computer equipment, between August 2016 and June 2017, and twice shoplifting materials for his bombs.
Prior to the sentencing, Elger had been subject to an interim hospital order while he underwent psychiatric assessment.
Previously, Winchester College said Elger had left before the motorway attacks in order to receive the "extensive medical support he required".