Damian Rzeszowski killed his wife Izabela, children Kinga (5) and Kacper (2), 56-year-old father-in-law Marek Garstka, family friend Marta de la Haye (34) and her five-year-old daughter Julia on 14 August.
In a statement released Monday, the States police issued a short statement saying: ‘The States of Jersey Police have been notified about the death of Mr Rzeszowski.
‘An investigation into his death will be carried out by the UK police. SOJP will not be issuing any further information.’
Rzeszowski died at 19.28 on 31 March, the force said. He was 37 years old.
Rzeszowski was serving a 30-year prison term in Full Sutton prison, a high-security jail in Yorkshire, after being convicted of the manslaughter of his victims. He was acquitted of murder, but pleaded guilty to the lesser charge on the grounds of diminished responsibility as he suffered from severe depression and psychotic symptoms. His defence team had argued that he was suffering from a severe abnormality of the brain at the time of the killings.