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.
"He didn't give off a vibe to him," David Melseaux told Sky News.
"He seemed like a really chilled person, it didn't really seem like he had anything wrong with him."
The 20-year-old trainee electrician lived in a flat directly above Prosper for over a decade.
"I have known him for years… It just goes to show you never know anyone's character really no matter how close you are to them."
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.
Mr Melseaux's family, who live on the ninth floor of their tower block, heard the shootings happen.
"They said the floor was shaking, they heard banging and screaming and it was just a very scary moment because they didn't know what was going on."
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.
He was arrested on a nearby street and a gun was found in bushes - officers had intercepted him before he could carry out the second phase of his attack.
Mr Melseaux told Sky News he couldn't believe what detectives said his neighbour planned to do.
"Why would someone want to do that?" he asked.
"Why would someone want to hurt innocent kids, something you should never do as a person - that's a line you should never cross."
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Parents outside the primary school in Luton told Sky News they were horrified by the prospect of what could have happened.
"It's stuff we expect from the (United) States not here… it's scary," one mother said.
One father said: "Having to sit your 11-year-old down and talk about someone with a gun at the school is not right.
"However we were able to convince them and reassure them that is it going to be OK."
The 19-year-old has been held on remand and will be sentenced next Wednesday.
(c) Sky News 2025: Neighbour of Luton teenager who murdered family speaks out after living above killer for decade
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