The older brother of the Manchester Arena suicide bomber and a convicted terrorist he visited prior to the attack have been ordered to give evidence at the inquiry.
Ismail Abedi previously refused to appear at the public inquiry because he said he feared he would "incriminate" himself.
His brother Salman Abedi killed 22 people and injured hundreds when he detonated a bomb at an Ariana Grande concert at Manchester Arena in the UK, on May 22, 2017.
The Manchester Arena inquiry is examining the circumstances of the attack and whether any opportunities to prevent it were missed.
On Wednesday, chairman of the inquiry Sir John Saunders said he would be issuing legal notices that will compel Ismail Abedi and convicted terrorist Abdalroauf Abdallah, who was visited in prison by Salman Abedi before the attack, to attend and answer questions.
Paul Greaney QC, counsel to the inquiry, said his team repeatedly contacted Ismael's solicitor and that he was given "every chance" to co-operate.
A letter sent to Ismael's legal team last October asking for him to engage with the inquiry said that he had "potentially important and significant evidence".
Mr Greaney said neither Ismael nor his solicitor had responded.
On Friday, a second letter was sent after a report raised issues about the radicalisation of Salman and his younger brother Hashem, who last year was sentenced to life in jail over the attack.
"There are matters within the report, in particular about the background and family life of Salman and Hashem where the chairman will be assisted with comments from your client," the letter said.
"The inquiry is a search for the truth and Ismail Abedi is in a unique position to assist with the investigation."
The inquiry heard that Salman was in contact with Abdallah in the months leading up to the attack.
Abdallah, 27, was sentenced to jail in 2016 after being found guilty of helping people travel to Syria to join ISIS.
The inquiry heard that Abdallah is a "witness with important evidence to give" and lawyers for the victims' families said a "failure to recognise the association between Salman Abedi and Abdalraouf Abdallah was a real missed opportunity".
Ahmed Taghdi, a friend of Salman, will also be told he must appear in person.
The inquiry continues.