No charges are to be brought over a two-day prison riot in which armed inmates took over a wing and smashed windows.
Riot-trained officers were sent to HMP The Mount, Hertfordshire, after trouble broke out on 31 July and 1 August.
Hertfordshire Police said it had exhausted all lines of inquiry, including reviewing CCTV footage.
Prisoners “must not feel they can do anything” without legal recourse, the Prison Officers’ Association said.
Police said reports were received on 31 July that prisoners had “threatened prison officers and caused damage to the prison” over a 10-hour period.
At the time, it was reported by BBC home affairs correspondent Danny Shaw that one wing and half of another were “lost”.
The following day it was reported to police that a prisoner had been assaulted by two inmates, while well-placed sources told the BBC armed prisoners had taken over the Nash wing of the prison.
A “nucleus of about 30 prisoners” was said to have been involved in the violence.
A Hertfordshire Police spokeswoman said a full investigation had been carried out and no charges were being brought.
“At this stage we have exhausted all lines of inquiry however, if any further information comes to light it will be fully investigated,” she said.
The Prison Officers Association said it was “disappointed” criminal proceedings were not being pursued.
“The incident resulted in significant damage to the prison and costs to the tax payer,” a spokesman said.
“Prisons have CCTV in all accommodation areas and so we are surprised evidence is not available to assist the police.”
Prison facts: HMP The Mount
The Mount Prison opened in 1987 as a young offenders institution and now houses more than 1,000 prisoners.
It was designed as a category C training prison built on the site of a former RAF station on the outskirts of Bovingdon village, Hertfordshire.
The prison is described as a “hybrid training and resettlement prison” for inmates in the final six months of their sentences.
A report in 2016 by the Independent Monitoring Board (IMB) found “all the ingredients” were in place for The Mount to suffer disorder such as has been experienced in other prisons:
- Staff shortages,
- Readily available drugs
- Mounting violence
The Crown Prosecution Service said it had not received a referral from police in relation to this matter.
The Ministry of Justice said it would not comment as it was a matter for the police.
The BBC also asked the Ministry of Justice in October for the cost of repairs to the prison but was told a final amount was not yet available.
This followed a previous Freedom of Information request for a report into the disturbance, which was refused.
On the second day of trouble at The Mount, about 30 inmates at Erlestoke prison in Wiltshire also became violent and four people were reported to have been taken to hospital.
In October, prison staff were attacked with pool balls at the high-security HMP Long Lartin in Worcestershire.
Police have previously brought charges against those in other prisons involved in disturbances. In September, six inmates were convicted for their part in a 15-hour riot at HMP Birmingham in December 2016.