The headteacher of Langdon Park School has said that "out of all his students", Amjol Amol was the "least likely" to be involved in violence. Chris Dunne added that the teenager had been working in Northern Ireland to promote peace in local communities.
A post-mortem is due to take place tomorrow.
More top news
Officers were given the touching letter at Brixton Station while they were on a reassurance patrol after yesterday's attack
The 52-year-old had previous convictions for violence but was not on the anti-terror watchlist.
In a statement Charlton Athletic said: Keith was a true hero who will be greatly missed by all the Charlton family and everyone at the club.