Suicide bomber Salman Abedi was groomed by a convicted terrorist before he carried out the Manchester Arena terrorist attack, an inquiry will hear on Tuesday.
Abedi killed 22 people and injured more than a thousand people when he blew himself up at the end of an Ariana Grande concert in May 2017.
On Tuesday, the Manchester Arena Inquiry, which is hearing evidence about the moments leading up to the attack, will hear evidence from extremism expert Dr Matthew Wilkinson.
Convicted terrorist Abdalraouf Abdallah, who Abedi had repeatedly visited in the months preceding the attack, will give his evidence on Wednesday.
Chairman of the inquiry, Sir John Saunders has ordered Abdallah to give evidence and rejected his plea to be excluded.
Abdallah, 28, who Dr Wilkinson says groomed Abedi, denies any involvement in the bombing.
He was jailed in May 2016 for preparing and funding acts of terrorism by helping others travel to Syria.
His lawyers had argued that compelling him to give evidence would breach his human rights.
Abdallah, from Manchester, was diagnosed with post-traumatic stress disorder after he became a paraplegic when he was fighting in Libya in 2011.
Sir John said Abdallah will be an "important witness".
"Dr Matthew Wilkinson, an expert on Islamist extremism, has suggested that Mr Abdallah exposed Salman Abedi to Islamist extremist views and played a role in radicalising him," he ruled.
"This is a view I intend to test in evidence with the expert. I would also wish for there to be the opportunity to explore relevant issues with Mr Abdallah himself. "
The inquiry has been told Abdallah has been advised by his legal team not to answer any questions.
The Manchester Arena inquiry is examining the circumstances of the attack and whether any opportunities to prevent it were missed.
Abedi, 22, died after detonating the explosive in a foyer of the arena at the end of the concert.
His younger brother, Hashem Abedi, was convicted last year on 22 counts of murder, one count of attempted murder and one count of conspiracy to cause an explosion.
He was sentenced to jail for a minimum of 55 years.
The inquiry continues.