A former prisoner who was jailed for shooting a car dealer in Rickmansworth said attempts were made to radicalise him while he was in prison.
Kevin Lane, 49, was jailed after he shot dead car dealer Robert Magill as he walked his dog in 1996.
He told the BBC he saw many vulnerable inmates pressured to convert and carry out attacks on other prisoners at jails including HMP Woodhill and HMP Frankland.
He said Islamist extremists targeted either influential inmates or those of "lower intellect" who were "easily impressionable".
"I have seen many attacks within the prison system," he said.
"One man boiled fat and poured it over someone's head because of an insult to Islam."
Last week's terror attack at Westminster has raised questions over how and why Khalid Masood was radicalised.
Security services currently have no clear indication, but Masood - born Adrian Elms - had spent much of his adult life in and out of jail.
A spokesman for the Ministry of Justice said: "The allegations put forward by the former prisoner are historic."