Prosecutors are deciding whether to charge some 110 prisoners suspected of being involved in riots which caused damage to a Yorkshire jail costing more than £1m.
South Yorkshire Police has ended its six-month investigation into the violence at Moorland Prison, near Doncaster, in which three staff and two inmates suffered injuries.
It is understood a female prison worker had her jaw broken in the fighting and one prisoner was beaten so badly he had to spend several days in hospital.
Inmates started fires, overturned pool tables and smashed up showers, toilets, tables and lockers during rampages on three consecutive nights last November.
Detectives have passed their files to lawyers at the Crown Prosecution Service (CPS), who are expected to confirm in August how many of the suspects should face charges.
The officer leading the inquiry, Det Insp Dave Mayfield, said it taken a “massive” effort to collect and compile all the evidence.
“We have now put forward files for charging advice to the CPS,” he said. “I have had a team of more than 20 officers at times committed to it for roughly six months.
“In the first few weeks, during the initial search phase of the inquiry, we had about 40 officers working on it.
“Scenes of crime officers moved into the prison, looking for forensic evidence, as well as specialist search officers.”
The trouble is believed to have started when a prison officer was injured while trying to stop a fight.
It began on the A wing of Moorland’s young offenders’ block, which brings together lifers aged 18 to 21 from across the country.
On the second night, violence broke out on the block’s C wing, the section directly opposite A wing.
The young offenders’ block was relatively calm on the third night but trouble flared in an adult houseblock.
Prison officers in riot gear, known as Tornado response teams, were called in to regain control of the jail.
Det Insp Mayfield said: “The third night was completely different to the first two because it was in the adult block.
“Small fires were started during the first two, but there was a big fire on the third night and anything that could be smashed was smashed.
“Fittings were ripped off the walls. Protective mesh on the walkways to prevent people falling had been damaged and the whole place was strewn with debris.
“We spent about three weeks just clearing the scene and gathering evidence. There were then so many cells which we had to search.”
The prison was so badly damaged that up to 250 inmates had to be moved to other jails, which meant South Yorkshire officers had to travel to Durham, Liverpool, Hull and Birmingham to interview them.
The Yorkshire Post understands that some prisoners who were injured during the riots were reluctant to co-operate with the inquiry for fear of reprisals from fellow offenders.
Detectives also had to travel to other jails to question prison officers who witnessed the violence. In total, staff from 18 prisons helped deal with the incidents.
More than half a year on, much of the Moorland estate remains out of bounds.
Prison population figures published by the Ministry of Justice reveal that less than 60 per cent of the jail could be used to accommodate inmates last month.
The jail would have room for 763 inmates under normal circumstances, but at May 27 it was recommended that only 453 should be held there.
Speaking shortly after the riots, the region’s head of prisons, Steve Wagstaffe, told the Yorkshire Post that authorities might never know what triggered the violence.
He said most of the inmates had been in the prison for some time and it was unlikely that the trouble had been started by incoming prisoners.
“It seems that just one spark when you have got the right conditions is enough to cause trouble,” Mr Wagstaffe added.