The investigation into an attack on three prison officers at a high-security prison on Saturday remains ongoing, counter-terrorism police have said.
Three officers were taken to hospital with "life-threatening injuries" after being attacked by Hashem Abedi, one of the men responsible for the Manchester Arena bombing, according to the Prison Officers' Association (POA).
The Prison Service confirmed that three officers were treated in hospital after the incident at HMP Frankland in County Durham - one of whom has since been discharged.
Abedi, 28, was jailed for life with a minimum term of 55 years in 2020 for helping his brother carry out the 2017 suicide bombing.
Counter-terrorism policing north east is currently leading the investigation with support from Durham Constabulary.
On Sunday, the counter-terrorism unit confirmed two of the three officers remain in hospital "with serious injuries".
"Extensive enquiries continue to establish the full circumstances and explore any potential motivation," the statement said.
"We are keeping an open mind as we investigate to establish the facts."
The POA said the officers sustained injuries including burns, scalds and stab wounds, as they outlined that Abedi threw hot cooking oil over them and used "home made weapons" to stab them.
National chairman of the POA, Mark Fairhurst, said the condition of the two officers who remain in hospital has since "stabilised".
The incident took place at a separation centre within the prison - designed to hold those considered the most dangerous and extremist offenders.
Hashem Abedi was found guilty in 2020 of 22 counts of murder, attempted murder and conspiracy to cause an explosion likely to endanger life and sentenced to a minimum term of at least 55 years before he could be considered for parole.
His brother, Salman Abedi, carried out the suicide attack at the Manchester Arena in 2017.
Hashem Abedi was later convicted of a previous attack on two prison officers at Belmarsh Prison in south-east London along with two others.
Fairhurst has called for the "immediate" restriction and removal of cooking facilities from separation centres as well as greater protections for frontline staff.
"We are worried about the knock-on effects and copycat incidents," he said on Sunday.
The Ministry of Justice said there will be a full review into the incident.
"The government will do whatever it takes to keep our hardworking staff safe," the department said.
A spokesperson for the Prison Service said violence in prisons "will not be tolerated", adding they will push for the "strongest punishment" for attacks on staff.