The suspect in the Sgt Matt Ratana murder inquiry watched violent videos at school and was obsessed with firearms, the BBC has been told.
The BBC has identified the suspect as Louis De Zoysa, 23, from Norbury, who remains critically ill in hospital.
Sgt Ratana, 54, died in hospital in the early hours of Friday after being shot at Croydon Custody Centre.
A former schoolmate said Mr De Zoysa was "reclusive" but "used to brag about being on the dark web."
Mr De Zoysa, the eldest of five children, attended the John Fisher secondary school in nearby Purley between 2008 and 2015.
A school leaving book, obtained by the BBC, shows that peers described Mr De Zoysa as "good with weapons".
Mr De Zoysa used to boast about accessing the dark web using his laptop while at school, the ex-classmate said.
Speaking anonymously, he said: "He used to brag about he could see hit men and guns for sale" and "used to watch murder videos… ISIS videos and things like that".
The source added: "We'd have school break at lunchtimes and free periods. You've got kids either playing outside or doing their homework and he was in the corner watching these videos all the time."
Mr De Zoysa is also said to have attempted to design weapons himself.
"He also had a weird obsession for weapons and guns and would design them on school computers when we used to do design technology," the former pupil said.
The source said that classmates were unaware of Mr De Zoysa having autism, which has been reported, saying that "whilst he was always an odd kid who I don't think had many friends, he always came across as very intelligent".
In the school leaving book there is a single quote from Mr De Zoysa, saying: "Do what you must, I have already won".
It is understood the suspect was later referred to the government's Prevent programme, aimed at stopping people joining extremist groups and carrying out terrorist activities.
But sources said he was not the subject of an investigation and he was unknown to MI5.
Scotland Yard has said the shooting of Sgt Matt Ratana is not being treated as a terrorist incident.
Mr De Zoysa was born in Croydon into a Roman Catholic family of Sri Lankan and British descent.
The John Fisher School said it was not commenting.