Three prison officers have been attacked by Hashem Abedi, the brother of the Manchester Arena bomber.
The officers sustained life-threatening injuries on Saturday including burns, scalds and stab wounds in the attack at HMP Frankland in County Durham, the Prison Officers' Association said.
Abedi threw hot cooking oil over the officers and used "home made weapons" to stab them, the organisation said.
The Prison Service confirmed three officers have been treated in hospital after an attack by a prisoner, adding police were investigating.
Two men and a woman were injured, with the latter since discharged from hospital.
A prison officer at HMP Frankland told BBC News "staff are shaken by what's happened".
"It's a difficult day at the prison when colleagues are seriously hurt. You can't help asking yourself why you do this job when something like this happens," they added.
Abedi is the brother of Salman Abedi who carried out the Manchester Arena bombing which killed 22 people in 2017.
After Hashem Abedi, 28, was named by the Prison Officers' Association as being involved in the attack, a government source confirmed to the BBC he was the prisoner involved.
Abedi was found guilty of 22 counts of murder, attempted murder and conspiracy to cause an explosion likely to endanger life in 2020 following the Manchester Arena attack