Two men are being questioned under terror laws over an attack on a prison officer at a maximum security jail.
The officer suffered stab wounds to his head, chest and face as the cells were unlocked at HMP Whitemoor in March, Cambridgeshire, on 9 January.
The men, aged 24 and 26, were arrested on Monday on suspicion of conspiracy to murder and preparation of a terrorist act, the Metropolitan Police said.
They are alleged to have attacked an officer with improvised weapons.
The attackers were wearing fake suicide belts, police said at the time.
Three prison officers and a nurse also suffered injuries as they rushed to the aid of their stabbed colleague.
Both the arrested men are in custody in a London police station while inquiries continue.
HMP Whitemoor houses more than 400 Category A and B prisoners on three wings, including a number of the highest-risk inmates.
In February last year, a "small number" of prison staff there needed medical treatment after violence broke out.