Two men are being questioned under terror laws over an attack on a prison officer at a maximum security jail.
Four officers and a nurse were injured as 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.
Both men are in custody in a London police station while inquiries continue, a spokesman added.
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.