Prosper, 19, pleaded guilty on Monday to murdering his mother Juliana Falcon, his 16-year-old brother Kyle Prosper and 13-year-old sister Giselle Prosper at the family home in Luton last September.
Triple killer Nicholas Prosper, who also planned to attack a primary school, gave no clues about what he was going to do, according to a neighbour.
After his guilty pleas, Bedfordshire Police said officers were in no doubt that the 19-year-old also planned to attack his old primary school in Luton.
Mr Melseaux told Sky News he couldn't believe what detectives said his neighbour planned to do.
Parents outside the primary school in Luton told Sky News they were horrified by the prospect of what could have happened.
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