The teenager who threw a six-year-old boy from a viewing platform at the Tate Modern was recorded telling carers a year earlier that he planned to push someone off a high building.
Jonty Bravery, 18, has admitted pushing the French boy from a 10th-floor viewing platform at the London gallery on August 4 last year.
It has now emerged that he had told carers he planned to kill someone by pushing them off a tall landmark.
The recording, obtained through a joint investigation by BBC News and the Daily Mail, features a voice believed to be Bravery saying: “I've got it in my head, a way to, a way to kill somebody.”
A witness to the horrifying incident has previously told how Bravery had blamed social services for his actions.
In the recording the autistic teenager describes how killing someone would mean he would go to prison instead of being in council care.
He says he would arrange to meet with someone and visit landmarks in central London - “as long as it’s a high thing”.
“And then push one of – push somebody off it,” he adds. “And I know for a fact they'll die from falling from the 100 feet.”
In interviews, one of Bravery’s carers described how carers could ‘never say no’ to Bravery because he would become aggressive and things would get out of hand, and said opportunities had been missed to stop the teen.
Bravery, of Ealing in west London, admitted attempted murder in December at the Old Bailey and is due to be sentenced this month.
His six-year-old victim, who was visiting London with his family at the time and is now recovering in France, suffered a bleed to the brain, fractures to his spine and broken legs and arms in the fall.
Bravery, who has autistic spectrum disorder and obsessive compulsive disorder, was under the care of Hammersmith and Fulham council in London, who reportedly subcontracted his care to private care provider Spencer and Arlington.
Responding to the BBC and Daily Mail’s investigation and the recording of Bravery saying he planned to kill someone, Spencer & Arlington told the BBC they had “no knowledge or records of the disclosure”.
In a statement, it said there was “absolutely no evidence” that Bravery “may have told his carers of his plan”.
But the company said it recognised the “gravity” of the claim and has reported the concerns to the Care Quality Commission (CQC).
Hammersmith and Fulham Council told the Daily Mail a serious case review is underway which will look at what happened and “the role played by all the different agencies involved”.